SoftEther: fix two heap-buffer-overflows

The first change is a proper (hopefully) fix for the bug reported in
8b6a00f.
The second one is related to:
```
==15096==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60f0000d7a00 at pc 0x55a2c593bd0b bp 0x7ffc92021cd0 sp 0x7ffc92021478
READ of size 3 at 0x60f0000d7a00 thread T0
    #0 0x55a2c593bd0a in strncmp (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x56fd0a) (BuildId: ee8631c0950a8cded5ba60c17f09709bbebbe5d8)
    ntop#1 0x55a2c5d1d9f9 in dissect_softether_host_fqdn /home/ivan/svnrepos/nDPI/src/lib/protocols/softether.c:249:9
    ntop#2 0x55a2c5d1b55b in ndpi_search_softether /home/ivan/svnrepos/nDPI/src/lib/protocols/softether.c:348:9
    ntop#3 0x55a2c5b0e9c5 in check_ndpi_detection_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5407:6
    ntop#4 0x55a2c5b0f78b in check_ndpi_udp_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5443:10
    ntop#5 0x55a2c5b0f12c in ndpi_check_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5476:12
    ntop#6 0x55a2c5b20f39 in ndpi_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:6303:15
    ntop#7 0x55a2c5a3014c in packet_processing /home/ivan/svnrepos/nDPI/example/reader_util.c:1600:31
    ntop#8 0x55a2c5a29062 in ndpi_workflow_process_packet /home/ivan/svnrepos/nDPI/example/reader_util.c:2170:10
    ntop#9 0x55a2c59e51a2 in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader.c:107:7
    ntop#10 0x55a2c590acb2 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x53ecb2) (BuildId: ee8631c0950a8cded5ba60c17f09709bbebbe5d8)
    ntop#11 0x55a2c590a3c5 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x53e3c5) (BuildId: ee8631c0950a8cded5ba60c17f09709bbebbe5d8)
    ntop#12 0x55a2c590c0f6 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x5400f6) (BuildId: ee8631c0950a8cded5ba60c17f09709bbebbe5d8)
    ntop#13 0x55a2c590c663 in fuzzer::Fuzzer::Loop(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x540663) (BuildId: ee8631c0950a8cded5ba60c17f09709bbebbe5d8)
    ntop#14 0x55a2c58faff2 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x52eff2) (BuildId: ee8631c0950a8cded5ba60c17f09709bbebbe5d8)
    ntop#15 0x55a2c5923c82 in main (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x557c82) (BuildId: ee8631c0950a8cded5ba60c17f09709bbebbe5d8)
    ntop#16 0x7f504ab98082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
    ntop#17 0x55a2c58efb1d in _start (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x523b1d) (BuildId: ee8631c0950a8cded5ba60c17f09709bbebbe5d8)

0x60f0000d7a00 is located 0 bytes to the right of 176-byte region [0x60f0000d7950,0x60f0000d7a00)
```
Found by oss-fuzzer
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=49736

src/lib/protocols/softether.c

@@ -98,7 +98,7 @@ static size_t dissect_softether_type(enum softether_value_type t,
 
       v->value.ptr.raw = payload + 4;
       u_int32_t siz = ntohl(get_u_int32_t(payload, 0));
-      if(payload_len < siz + 3)
+      if(siz == 0 || payload_len < siz + 3)
 	return 0;
 
       if(t == VALUE_DATA)
@@ -263,9 +263,6 @@ static int dissect_softether_ip_port(struct ndpi_flow_struct *flow,
   if(ip_port_separator == NULL)    
     return 1;    
 
-  if(ip_port_separator < (char const *)packet->payload + NDPI_STATICSTRING_LEN("IP="))    
-    return 1;    
-
   ip_len = ndpi_min(sizeof(flow->protos.softether.ip) - 1,
                     ip_port_separator - (char const *)packet->payload -
                     NDPI_STATICSTRING_LEN("IP="));
@@ -275,16 +272,14 @@ static int dissect_softether_ip_port(struct ndpi_flow_struct *flow,
           ip_len);
   flow->protos.softether.ip[ip_len] = '\0';
 
-  if(ip_port_separator < (char const *)packet->payload +
-     NDPI_STATICSTRING_LEN("IP=") + NDPI_STATICSTRING_LEN(",PORT="))
-    return 1;    
+  if (packet->payload_packet_len < (ip_port_separator - (char const *)packet->payload) +
+                                   NDPI_STATICSTRING_LEN(",PORT="))
+    return 1;
 
   port_len = ndpi_min(sizeof(flow->protos.softether.port) - 1,
-                      ip_port_separator - (char const *)packet->payload -
-                      NDPI_STATICSTRING_LEN("IP=") - NDPI_STATICSTRING_LEN(",PORT="));
-
-  strncpy(flow->protos.softether.port,
-	  ip_port_separator + NDPI_STATICSTRING_LEN(",PORT="),
+                      packet->payload_packet_len - (ip_port_separator - (char const *)packet->payload) -
+                      NDPI_STATICSTRING_LEN(",PORT="));
+  strncpy(flow->protos.softether.port, ip_port_separator + NDPI_STATICSTRING_LEN(",PORT="),
           port_len);
 
   flow->protos.softether.port[port_len] = '\0';