gtphub: Fix use after free on failure

Even if fclose fails the stream is inaccessible and the second fclose
might cause memory violation.

Linux manpage says:
Upon  successful  completion 0 is returned.  Otherwise, EOF is returned
and errno is set to indicate the error.  In either case any further
access (including another call to fclose()) to the stream results in
undefined behavior.

Fixes: CID#57958

openbsc/src/gprs/gtphub_main.c

@@ -166,8 +166,10 @@ static uint8_t next_restart_count(const char *path)
 		goto failed_to_write;
 	if (fprintf(f, "%" PRIu8 "\n", counter) < 2)
 		goto failed_to_write;
-	if (fclose(f))
+	if (fclose(f)) {
+		f = NULL;
 		goto failed_to_write;
+	}
 
 	umask(umask_was);