Use safer method to get string without hit

spacy/strings.pyx

@@ -251,6 +251,7 @@ cdef class StringStore:
 
     def _cleanup_stale_strings(self, excepted):
         """
+        excepted (list): Strings that should not be removed.
         RETURNS (keys, strings): Dropped strings and keys that can be dropped from other places
         """
         if self.hits.size() == 0:
@@ -262,7 +263,9 @@ cdef class StringStore:
         dropped_keys = []
         for i in range(self.keys.size()):
             key = self.keys[i]
-            value = self[key]
+            # Here we cannot use __getitem__ because it also set hit.
+            utf8str = <Utf8Str*>self._map.get(key)
+            value = decode_Utf8Str(utf8str)
             if self.hits.count(key) != 0 or value in excepted:
                 tmp.push_back(key)
             else:

spacy/tests/regression/test_issue1506.py

@@ -13,6 +13,9 @@ def test_issue1506():
         for _ in range(10001):
             yield "It's sentence produced by that bug."
 
+        for _ in range(10001):
+            yield "I erase some hbdsaj lemmas."
+
         for _ in range(10001):
             yield "I erase lemmas."
 

spacy/vocab.pyx

@@ -469,6 +469,7 @@ cdef class Vocab:
         for k in keys:
             del self._by_hash[k]
 
+        if len(strings) != 0:
             self._by_orth = PreshMap()