Fix crash on invalid type comment

Author: ilevkivskyi

src/com2ann.py

@@ -411,7 +411,7 @@ def split_function_comment(
     is transformed into: ['int', 'str'], 'None'.
     """
     typ, _ = split_sub_comment(comment)
-    if "->" not in typ:
+    if typ.count("->") != 1:
         if not silent:
             print("Invalid function type comment:", comment, file=sys.stderr)
         return None

src/test_com2ann.py

@@ -374,6 +374,20 @@ def load_cache(self) -> bool:
             """,
         )
 
+    def test_invalid_return_type(self) -> None:
+        self.check(
+            """
+            def load_cache(x):
+                # type: (str) -> bool -> invalid
+                pass
+            """,
+            """
+            def load_cache(x):
+                # type: (str) -> bool -> invalid
+                pass
+            """,
+        )
+
     def test_combined_annotations_single(self) -> None:
         self.check(
             """