gio/tests: Add non-NULL assertions to help static analysis

These prevent some false positives from the static analyser which are
caused by it not inspecting the invariants of
g_subprocess_communicate[_utf8]_finish() (i.e. that stdout and
stdout_str will always be set unless an error was returned).

They’re also good testing anyway.

Found by scan-build.

https://bugzilla.gnome.org/show_bug.cgi?id=113075

gio/tests/gsubprocess.c

@@ -583,10 +583,12 @@ on_communicate_complete (GObject               *proc,
 
   if (!data->is_utf8)
     {
+      g_assert (stdout != NULL);
       stdout_data = g_bytes_get_data (stdout, &stdout_len);
     }
   else
     {
+      g_assert (stdout_str != NULL);
       stdout_data = (guint8*)stdout_str;
       stdout_len = strlen (stdout_str);
     }