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
1 file changed