subprocess: avoid infinite loop in verify_disposition()

When performing the verify and building the error string there were two
possibilities of an infinite loop. The first is the missing twos-complement
to unset the bit in the filtered flags. The second is the lack of handling
G_SUBPROCESS_FLAGS_NONE which can return a valid GFlagsValue (and cannot
unset the bit since the value is zero).

This walks all known values in the GSubprocessFlags type class and check
if they are set. This has the benefit that we don't call needless functions
which walk the same table as well as avoiding mutating values to build
the error string.

https://bugzilla.gnome.org/show_bug.cgi?id=775913

gio/gsubprocesslauncher.c

@@ -79,13 +79,16 @@ verify_disposition (const gchar      *stream_name,
       if (n_bits)
         {
           GFlagsClass *class;
-          GFlagsValue *value;
+          guint i;
 
           class = g_type_class_peek (G_TYPE_SUBPROCESS_FLAGS);
-          while ((value = g_flags_get_first_value (class, filtered_flags)))
+
+          for (i = 0; i < class->n_values; i++)
             {
+              const GFlagsValue *value = &class->values[i];
+
+              if (filtered_flags & value->value)
                 g_string_append_printf (err, " %s", value->value_name);
-              filtered_flags &= value->value;
             }
 
           g_type_class_unref (class);