Fix a use-of-uninitialized-value error on PCRE code

This is likely a false positive, triggered by the fact that libpcre is
usually compiled without MASAN support.
It it was a real error, ASAN would complain loudly with a invalid-free
error at the end of the same function.

```
==83793==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x562296111174 in ndpi_compile_rce_regex /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:1631:3
    ntop#1 0x5622960e3e4a in ndpi_is_rce_injection /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:1636:5
    ntop#2 0x5622960de7cd in ndpi_validate_url /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:1741:12
    ntop#3 0x5622960dae45 in ndpi_dpi2json /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:1362:29
    ntop#4 0x5622960e2751 in ndpi_flow2json /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:1512:10
    ntop#5 0x562296033b0f in process_ndpi_collected_info /home/ivan/svnrepos/nDPI/example/reader_util.c:1310:9
    ntop#6 0x5622960501f9 in packet_processing /home/ivan/svnrepos/nDPI/example/reader_util.c:1659:2
    ntop#7 0x562296045aef in ndpi_workflow_process_packet /home/ivan/svnrepos/nDPI/example/reader_util.c:2202:10
    ntop#8 0x562295e85374 in ndpi_process_packet /home/ivan/svnrepos/nDPI/example/ndpiReader.c:3937:7
    ntop#9 0x7f1235053466  (/lib/x86_64-linux-gnu/libpcap.so.0.8+0x23466) (BuildId: b84c893ea2516d6fb2c1c6726b1fe93b3be78f61)
    ntop#10 0x7f1235041f67 in pcap_loop (/lib/x86_64-linux-gnu/libpcap.so.0.8+0x11f67) (BuildId: b84c893ea2516d6fb2c1c6726b1fe93b3be78f61)
    ntop#11 0x562295e53139 in runPcapLoop /home/ivan/svnrepos/nDPI/example/ndpiReader.c:4060:15
    ntop#12 0x562295e51e7f in processing_thread /home/ivan/svnrepos/nDPI/example/ndpiReader.c:4130:3
    ntop#13 0x7f1234e53608 in start_thread /build/glibc-SzIz7B/glibc-2.31/nptl/pthread_create.c:477:8
    ntop#14 0x7f1234d2f132 in __clone /build/glibc-SzIz7B/glibc-2.31/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S:95

  Uninitialized value was created by an allocation of 'pcreErrorStr' in the stack frame
    #0 0x5622961108a6 in ndpi_compile_rce_regex /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:1603:3
```

src/lib/ndpi_utils.c

@@ -1640,7 +1640,7 @@ static int ndpi_is_xss_injection(char* query) {
 #ifdef HAVE_PCRE
 
 static void ndpi_compile_rce_regex() {
-  const char *pcreErrorStr;
+  const char *pcreErrorStr = NULL;
   int pcreErrorOffset;
 
   for(int i = 0; i < N_RCE_REGEX; i++) {