Don't fail during ValueRecord copy if src has more items

We drop hinting by simply changing ValueFormat, without cleaning
up the actual ValueRecords. This was causing failure at this assert
if font was subsetted without hinting and then passed to mutator.

Lib/fontTools/ttLib/tables/otBase.py

@@ -894,7 +894,8 @@ class ValueRecord(object):
 					setattr(self, name, None if isDevice else 0)
 			if src is not None:
 				for key,val in src.__dict__.items():
-					assert hasattr(self, key)
+					if not hasattr(self, key):
+						continue
 					setattr(self, key, val)
 		elif src is not None:
 			self.__dict__ = src.__dict__.copy()