RTITCP: fix heuristic dissector

Ensure that 8 bytes are captured before checking magic content

Change-Id: I44465157b97169a5dc1c359a8e06b56e9572f8d5
Reviewed-on: https://code.wireshark.org/review/13423
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>

epan/dissectors/packet-rtitcp.c

@@ -764,6 +764,9 @@ static gint dissect_rtitcp(tvbuff_t *tvb, packet_info *pinfo,
 
     gboolean desegmentation = TRUE;
 
+    if (tvb_captured_length(tvb) < 8)
+        return 0;
+
     /* Check if the RTITCP_MAGIC_NUMBER is here */
     if (tvb_get_ntohl(tvb, 4) != RTITCP_MAGIC_NUMBER)
         return 0;