Fix a Use-of-uninitialized-value error

```
==1899==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x59a0a2567a0b in dissect_l2 ndpi/src/lib/third_party/src/fuzz/pl7m.c:291:6
    #1 0x59a0a25663ea in dissect_do ndpi/src/lib/third_party/src/fuzz/pl7m.c:711:7
    #2 0x59a0a25663ea in __deserialize_from_fd ndpi/src/lib/third_party/src/fuzz/pl7m.c:1211:8
    #3 0x59a0a2564147 in __deserialize ndpi/src/lib/third_party/src/fuzz/pl7m.c:1241:9
    #4 0x59a0a2564147 in pl7m_mutator ndpi/src/lib/third_party/src/fuzz/pl7m.c:1296:6
```
Found by oss-fuzz (via nDPI)

pl7m.c

@@ -284,7 +284,7 @@ static int dissect_l2(int datalink_type, struct m_pkt *p)
 		break;
 
 	case DLT_PPI:
-		if (data_len < l2_offset + 4)
+		if (data_len < l2_offset + 8)
 			return -1;
 		header_length = le16toh(*(u_int16_t *)&data[l2_offset + 2]);
 		dlt = le32toh(*(u_int32_t *)&data[l2_offset + 4]);