common/dir: Clarify the nullability of some variables

flatpak_decompose_ref() can fail, but in this situation we know it won’t
since find_matching_refs() checks it for us and hence guarantees that it
only returns decomposable refs. Add some assertions to make this clearer
and to shut Coverity up.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

common/flatpak-dir.c

@@ -6833,6 +6833,7 @@ flatpak_dir_remote_list_refs (FlatpakDir       *self,
   return TRUE;
 }
 
+/* Guarantees to return refs which are decomposable. */
 static GPtrArray *
 find_matching_refs (GHashTable *refs,
                     const char   *opt_name,
@@ -6945,6 +6946,7 @@ find_matching_ref (GHashTable *refs,
             {
               char *current_ref = g_ptr_array_index (matched_refs, j);
               g_auto(GStrv) parts = flatpak_decompose_ref (current_ref, NULL);
+              g_assert (parts != NULL);
 
               if (g_strcmp0 (opt_default_branch, parts[3]) == 0)
                 return g_strdup (current_ref);
@@ -6958,6 +6960,7 @@ find_matching_ref (GHashTable *refs,
       for (j = 0; j < matched_refs->len; j++)
         {
           g_auto(GStrv) parts = flatpak_decompose_ref (g_ptr_array_index (matched_refs, j), NULL);
+          g_assert (parts != NULL);
           if (j != 0)
             g_string_append (err, ", ");