gio-test: fix leak & maybe-uninitialized warning
gio-test: fix leak & maybe-uninitialized warning

GCC 8 on F29 complains:

../tests/gio-test.c: In function ‘main’:
../tests/gio-test.c:375:7: warning: ‘id’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       g_free (id);
       ^~~~~~~~~~~

In the normal case, when run without arguments, 'id' will be assigned
exactly once, so all is fine. If run with argument '0', 'id' will never
be assigned, so the warning is legit; but in that case the test program
will never exit. If run with any argument greater than 1, 'id' will be
assigned more than once but only the last incarnation will be freed.

Tweak the scope of the variable to match its use, and arrange for it to
be freed when its watch is destroyed.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib