utils: add NULL assignment after free

This eliminates double free if next strdup fails.

refs #2147

Change-Id: Ib6f785b4401144c8fa88feb6c3244de3332e0f5e

src/utils/common/netio.c

@@ -162,6 +162,7 @@ static void get_addr_str(const struct sockaddr_storage *ss,
 
 	// Free previous string if any.
 	free(*dst);
+	*dst = NULL;
 
 	// Write formated information string.
 	int ret = snprintf(buf, sizeof(buf), "%s#%u(%s)", addr, port,