Avoid race condition when receiving signal during shutdown

If we receive a TERM signal while we're shutting down, we
can access a free'd pcap_t while trying to call pcap_breakloop().
We avoid this by setting the "pd" to NULL after freeing it, and don't
try to use it if it's NULL.

Author: fenner

tcpdump.c

@@ -2783,6 +2783,7 @@ DIAG_ON_ASSIGN_ENUM
 			info(1);
 		}
 		pcap_close(pd);
+		pd = NULL;
 		if (VFileName != NULL) {
 			ret = get_next_file(VFile, VFileLine);
 			if (ret) {
@@ -3062,7 +3063,8 @@ cleanup(int signo _U_)
 	 * to do anything with standard I/O streams in a signal handler -
 	 * the ANSI C standard doesn't say it is).
 	 */
-	pcap_breakloop(pd);
+	if (pd)
+		pcap_breakloop(pd);
 }
 
 /*