Don't allocate a token run if the text has no more characters.

Based on this commit by JensOgorek:
zauberzeug#6

Author: ianh

src/x-tokenize.h

@@ -236,6 +236,8 @@ static bool OWL_DONT_INLINE
 owl_default_tokenizer_advance(struct owl_default_tokenizer *tokenizer,
  struct owl_token_run **previous_run)
 {
+    if (tokenizer->text[tokenizer->offset] == '\0')
+        return false;
     struct owl_token_run *run = malloc(sizeof(struct owl_token_run));
     if (!run)
         return false;