gtphub_test: fix compilation error on gcc 10.2.0

Ensure dump && hex can't be zero.

gtphub_test.c: In function ‘_reply_is.constprop’:
gtphub_test.c:535:3: error: ‘%s’ directive argument is null [-Werror=format-overflow=]

Change-Id: Id27bf46855a228935fe706584f9b27cf3facf623

tests/gtphub/gtphub_test.c

@@ -529,6 +529,8 @@ static int _reply_is(const char *hex, const char *file, int line)
 	const char *dump = osmo_hexdump_nospc(reply_buf, len);
 	int cmp = strcmp(dump, hex);
 
+	OSMO_ASSERT(dump && hex);
+
 	if (cmp != 0) {
 		printf("\n%s:%d: reply_is(): MISMATCH\n"
 		       "  expecting:\n'%s'\n"