Add guard against a rare crash in the C tokenizer

CHANGELOG

@@ -1,6 +1,7 @@
-v0.6 (unreleased):
+v0.5.4 (unreleased):
 
-- ...
+- Fixed an unlikely crash in the C tokenizer when interrupted while parsing
+  a heading.
 
 v0.5.3 (released March 30, 2019):
 

docs/changelog.rst

@@ -1,13 +1,14 @@
 Changelog
 =========
 
-v0.6
+v0.5.4
 ----
 
 Unreleased
 (`changes <https://github.com/earwig/mwparserfromhell/compare/v0.5.3...develop>`__):
 
-- ...
+- Fixed an unlikely crash in the C tokenizer when interrupted while parsing
+  a heading.
 
 v0.5.3
 ------

mwparserfromhell/parser/ctokenizer/tok_parse.c

@@ -813,6 +813,9 @@ static int Tokenizer_parse_heading(Tokenizer* self)
         self->global ^= GL_HEADING;
         return 0;
     }
+    if (!heading) {
+        return -1;
+    }
 #ifdef IS_PY3K
     level = PyLong_FromSsize_t(heading->level);
 #else
@@ -892,6 +895,9 @@ static HeadingData* Tokenizer_handle_heading_end(Tokenizer* self)
         self->head = reset + best - 1;
     }
     else {
+        if (!after) {
+            return NULL;
+        }
         for (i = 0; i < best; i++) {
             if (Tokenizer_emit_char(self, '=')) {
                 Py_DECREF(after->title);