reader_util: fix parsing of MPLS packets (ntop#1467)

```
==20492==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60300000578c at pc 0x55c47455e3ea bp 0x7ffc62ca1eb0 sp 0x7ffc62ca1ea8
READ of size 4 at 0x60300000578c thread T0
    #0 0x55c47455e3e9 in ndpi_workflow_process_packet /home/ivan/svnrepos/nDPI/example/reader_util.c:1840:16
    #1 0x55c47451b9cd in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader.c:107:7
    #2 0x55c47451c1ab in main /home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader.c:179:17
    #3 0x7f661b50e0b2 in __libc_start_main /build/glibc-sMfBJT/glibc-2.31/csu/../csu/libc-start.c:308:16
    #4 0x55c47445b54d in _start (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader_with_main+0x61054d) (BuildId: eba4bb4cd43b7101e4f0028ec0fb79087bae0e37)

0x60300000578d is located 0 bytes to the right of 29-byte region [0x603000005770,0x60300000578d)

```

example/reader_util.c

@@ -1837,6 +1837,8 @@ struct ndpi_proto ndpi_workflow_process_packet(struct ndpi_workflow * workflow,
 
   case ETH_P_MPLS_UNI:
   case ETH_P_MPLS_MULTI:
+    if(ip_offset+4 >= (int)header->caplen)
+      return(nproto);
     mpls.u32 = *((uint32_t *) &packet[ip_offset]);
     mpls.u32 = ntohl(mpls.u32);
     workflow->stats.mpls_count++;