dir: Consistently initialize g_autofree variables

All g_autofree and g_autoptr variables should be initialized, either
with a value or NULL, and some gcc versions warn if they are not.

This particular instance was harmless, because we initialized regexp
to either NULL or a valid non-NULL value as the first statement of the
function, so there was no way to return before regexp was initialized;
but if we introduced an early-return such as a g_return_val_if_fail
before the current first statement, then that early return would have
caused uninitialized stack contents to be freed, likely resulting in a
crash.

Signed-off-by: Simon McVittie <smcv@collabora.com>

common/flatpak-dir.c

@@ -4402,7 +4402,7 @@ flatpak_dir_config_append_pattern (FlatpakDir *self,
                                    GError    **error)
 {
   g_autoptr(GPtrArray) patterns = flatpak_dir_get_config_patterns (self, key);
-  g_autofree char *regexp;
+  g_autofree char *regexp = NULL;
   gboolean already_present;
   g_autofree char *merged_patterns = NULL;