Avoid clobbering existing lemmas

spacy/morphology.pyx

@@ -55,7 +55,7 @@ cdef class Morphology:
         # Add a 'null' tag, which we can reference when assign morphology to
         # untagged tokens.
         self.rich_tags[self.n_tags].id = self.n_tags
- 
+
         self._cache = PreshMapArray(self.n_tags)
         self.exc = {}
         if exc is not None:
@@ -68,7 +68,8 @@ cdef class Morphology:
 
     cdef int assign_untagged(self, TokenC* token) except -1:
         '''Set morphological attributes on a token without a POS tag.'''
-        token.lemma = self.lemmatize(0, token.lex.orth, {})
+        if token.lemma == 0:
+            token.lemma = self.lemmatize(0, token.lex.orth, {})
 
     cdef int assign_tag(self, TokenC* token, tag) except -1:
         if isinstance(tag, basestring):