Fix a fix: defrag OOM condition

** CID 1257764:  Dereference after null check  (FORWARD_NULL)
/src/defrag.c: 291 in Defrag4Reassemble()

** CID 1257763:  Dereference after null check  (FORWARD_NULL)
/src/defrag.c: 409 in Defrag6Reassemble()

In the error case 'rp' can be both NULL or non-NULL.

src/defrag.c

@@ -358,7 +358,8 @@ done:
 error_remove_tracker:
     tracker->remove = 1;
     DefragTrackerFreeFrags(tracker);
-    PacketFreeOrRelease(rp);
+    if (rp != NULL)
+        PacketFreeOrRelease(rp);
     return NULL;
 }
 
@@ -480,7 +481,8 @@ done:
 error_remove_tracker:
     tracker->remove = 1;
     DefragTrackerFreeFrags(tracker);
-    PacketFreeOrRelease(rp);
+    if (rp != NULL)
+        PacketFreeOrRelease(rp);
     return NULL;
 }