sockettest: testMaskNetwork: use g_auto

sockettest: testMaskNetwork: use g_auto

This has the added benefit of 'gotnet' only being freed after
it was possibly used in the output string.

../src/internal.h:519:27: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  519 | # define fprintf(fh, ...) g_fprintf(fh, __VA_ARGS__)
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~
../tests/sockettest.c:194:9: note: in expansion of macro ‘fprintf’
  194 |         fprintf(stderr, "Expected %s, got %s\n", networkstr, gotnet);
      |         ^~~~~~~

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reported-by: Jaroslav Suchanek <jsuchane@redhat.com>
Fixes: ba08c5932e556aa4f5101357127a6224c40e5ebe
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>