HTTP: fix classification in some corner cases

Initializing `low->detected_protocol_stack[1]` but not
`low->detected_protocol_stack[0]` lead to *very* strange errors.
Oss-fuzzer, exploiting this bug, has been able to crash the application
or to leak some memory

```
==19775==ERROR: AddressSanitizer: SEGV on unknown address (pc 0x5624c0dd1802 bp 0x5624c197e848 sp 0x7fff4b4db690 T0)
==19775==The signal is caused by a READ memory access.
==19775==Hint: this fault was caused by a dereference of a high value address (see register values below).  Disassemble the provided pc to learn which register was used.
    #0 0x5624c0dd1802 in __asan::Allocator::Deallocate(void*, unsigned long, unsigned long, __sanitizer::BufferedStackTrace*, __asan::AllocType) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader_with_main+0x522802) (BuildId: 9a1ed9efcaf47e345767c86520372d28e31ca1aa)
    ntop#1 0x5624c0e53ea6 in __interceptor_free (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader_with_main+0x5a4ea6) (BuildId: 9a1ed9efcaf47e345767c86520372d28e31ca1aa)
    ntop#2 0x5624c0ebb474 in free_wrapper /home/ivan/svnrepos/nDPI/example/reader_util.c:330:3
    ntop#3 0x5624c0f75561 in ndpi_free /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:240:7
    ntop#4 0x5624c0faa921 in ndpi_free_flow_data /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:4915:2
    ntop#5 0x5624c0f755ef in ndpi_free_flow /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:8076:5
    ntop#6 0x5624c0f755c4 in ndpi_flow_free /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:253:5
    ntop#7 0x5624c0eba981 in ndpi_free_flow_info_half /home/ivan/svnrepos/nDPI/example/reader_util.c:305:25
    ntop#8 0x5624c0ecb4bc in process_ndpi_collected_info /home/ivan/svnrepos/nDPI/example/reader_util.c:1317:5
    ntop#9 0x5624c0edca36 in packet_processing /home/ivan/svnrepos/nDPI/example/reader_util.c:1639:2
    ntop#10 0x5624c0ed5172 in ndpi_workflow_process_packet /home/ivan/svnrepos/nDPI/example/reader_util.c:2182:10
    ntop#11 0x5624c0e90a80 in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader.c:107:7
    ntop#12 0x5624c0e90bbb in main /home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader.c:179:17
    ntop#13 0x7f580731f082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
    ntop#14 0x5624c0dcf52d in _start (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader_with_main+0x52052d) (BuildId: 9a1ed9efcaf47e345767c86520372d28e31ca1aa)
```

```
==17492==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 5 byte(s) in 1 object(s) allocated from:
    #0 0x55b799b0f01e in malloc (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet_with_main+0x56101e) (BuildId: d2adbfb29a6eda6dc59fdfb8930d7e6496ac7b8b)
    ntop#1 0x55b799bb3bf4 in ndpi_malloc /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:212:46
    ntop#2 0x55b799bb3f28 in ndpi_strdup /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:279:13
    ntop#3 0x55b799c8b9bf in processCertificateElements /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:683:39
    ntop#4 0x55b799c81e89 in processCertificate /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:835:7
    ntop#5 0x55b799c8d500 in processTLSBlock /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:889:13
    ntop#6 0x55b799ca80f1 in ndpi_search_tls_tcp /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:1034:2
    ntop#7 0x55b799ca3be2 in ndpi_search_tls_wrapper /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:2475:5
    ntop#8 0x55b799bf9345 in check_ndpi_detection_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5414:6
    ntop#9 0x55b799bf9d57 in check_ndpi_tcp_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5462:12
    ntop#10 0x55b799bf99f7 in ndpi_check_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5481:12
    ntop#11 0x55b799c0b838 in ndpi_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:6308:15
    ntop#12 0x55b799b4b87e in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet.c:29:5
```

```
==17511==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 1 byte(s) in 1 object(s) allocated from:
    #0 0x55aed645e01e in malloc (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet_with_main+0x56101e) (BuildId: d2adbfb29a6eda6dc59fdfb8930d7e6496ac7b8b)
    ntop#1 0x55aed6502bf4 in ndpi_malloc /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:212:46
    ntop#2 0x55aed6502f28 in ndpi_strdup /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:279:13
    ntop#3 0x55aed65ea0c6 in processClientServerHello /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:2153:34
    ntop#4 0x55aed65dbe77 in processTLSBlock /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:867:5
    ntop#5 0x55aed65f70f1 in ndpi_search_tls_tcp /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:1034:2
    ntop#6 0x55aed65f2be2 in ndpi_search_tls_wrapper /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:2475:5
    ntop#7 0x55aed6548345 in check_ndpi_detection_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5414:6
    ntop#8 0x55aed6548d57 in check_ndpi_tcp_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5462:12
    ntop#9 0x55aed65489f7 in ndpi_check_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5481:12
    ntop#10 0x55aed655a838 in ndpi_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:6308:15
    ntop#11 0x55aed649a87e in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet.c:29:5
```

See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=49844
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=49842
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=49906
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=49915

src/lib/protocols/http.c

@@ -758,7 +758,8 @@ static void check_content_type_and_change_protocol(struct ndpi_detection_module_
     if(flow->detected_protocol_stack[1] == NDPI_PROTOCOL_UNKNOWN) {
       /* Avoid putting as subprotocol a "core" protocol such as SSL or DNS */
       if(ndpi_struct->proto_defaults[flow->guessed_protocol_id].subprotocol_count == 0) {
-	if(flow->detected_protocol_stack[0] == NDPI_PROTOCOL_UNKNOWN) {
+	if(flow->detected_protocol_stack[0] == NDPI_PROTOCOL_UNKNOWN &&
+	   flow->guessed_host_protocol_id != NDPI_PROTOCOL_UNKNOWN) {
 	  flow->detected_protocol_stack[0] = flow->guessed_host_protocol_id;
 	  flow->detected_protocol_stack[1] = flow->guessed_protocol_id;
         }