[subset] always return a valid face from hb_subset_preprocess.

src/hb-subset-input.cc

@@ -464,6 +464,8 @@ HB_EXTERN hb_face_t *
 hb_subset_preprocess (hb_face_t *source)
 {
   hb_subset_input_t* input = hb_subset_input_create_or_fail ();
+  if (!input)
+    return source;
 
   hb_set_clear (hb_subset_input_set(input, HB_SUBSET_SETS_UNICODE));
   hb_set_invert (hb_subset_input_set(input, HB_SUBSET_SETS_UNICODE));
@@ -505,6 +507,11 @@ hb_subset_preprocess (hb_face_t *source)
   hb_face_t* new_source = hb_subset_or_fail (source, input);
   hb_subset_input_destroy (input);
 
+  if (!new_source) {
+    DEBUG_MSG (SUBSET, nullptr, "Preprocessing failed due to subset failure.");
+    return source;
+  }
+
   return new_source;
 }
 #endif