Fix an issue around completions in comments before strings

jedi/api/completion.py

@@ -283,6 +283,9 @@ class Completion:
 
 
 def _extract_string_while_in_string(leaf, position):
+    if position < leaf.start_pos:
+        return None, None
+
     if leaf.type == 'string':
         match = re.match(r'^\w*(\'{3}|"{3}|\'|")', leaf.value)
         quote = match.group(1)

test/test_api/test_completion.py

@@ -125,6 +125,10 @@ def test_in_comment(Script):
     assert not Script("max_attr_value = int(2) # Cast to int for spe").completions()
 
 
+def test_in_comment_before_string(Script):
+    assert not Script(" # Foo\n'asdf'", line=1).completions()
+
+
 def test_async(Script, environment):
     if environment.version_info < (3, 5):
         pytest.skip()