[face] Add hb_face_create_or_fail()

New API:
+ hb_face_create_or_fail()

docs/harfbuzz-sections.txt

@@ -357,6 +357,7 @@ hb_directwrite_face_get_font_face
 hb_face_count
 hb_face_t
 hb_face_create
+hb_face_create_or_fail
 hb_reference_table_func_t
 hb_face_create_for_tables
 hb_face_get_empty

src/hb-face.cc

@@ -263,6 +263,34 @@ hb_face_create (hb_blob_t    *blob,
   return face;
 }
 
+/**
+ * hb_face_create_or_fail:
+ * @blob: #hb_blob_t to work upon
+ * @index: The index of the face within @blob
+ *
+ * Like hb_face_create(), but returns `NULL` if the blob data
+ * contains no usable font face at the specified index.
+ *
+ * Return value: (transfer full): The new face object, or `NULL` if
+ * no face is found at the specified index
+ *
+ * XSince: REPLACEME
+ **/
+hb_face_t *
+hb_face_create_or_fail (hb_blob_t    *blob,
+			unsigned int  index)
+{
+  unsigned num_faces = hb_face_count (blob);
+  if (index >= num_faces)
+    return nullptr;
+
+  hb_face_t *face = hb_face_create (blob, index);
+  if (hb_object_is_immutable (face))
+    return nullptr;
+
+  return face;
+}
+
 /**
  * hb_face_get_empty:
  *

src/hb-face.h

@@ -59,6 +59,10 @@ HB_EXTERN hb_face_t *
 hb_face_create (hb_blob_t    *blob,
 		unsigned int  index);
 
+HB_EXTERN hb_face_t *
+hb_face_create_or_fail (hb_blob_t    *blob,
+			unsigned int  index);
+
 /**
  * hb_reference_table_func_t:
  * @face: an #hb_face_t to reference table for