Segfault when grammar entry rule doesn't exists

[mingodad]

Describe the bug
Segfault when grammar entry rule doesn't exists

To Reproduce
Steps to reproduce the behavior:

1. Using the README json example execute peppa parse -G json.peg -e entry2 data.json
2. See the segfault

Desktop (please complete the following information):

• OS: linux Ubuntu 18.04 64bits

This seems to fix the problem (line numbers are from my code with other changes):

@@ -3194,9 +3330,9 @@ P4_GetErrorMessage(P4_Source* source) {
         return NULL;
 
     memset(source->errmsg, 0, sizeof(source->errmsg));
     sprintf(source->errmsg, "line %zu:%zu, expect %s",
             source->error.lineno, source->error.offset,
-            source->error.rule->name);
+            source->error.rule ? source->error.rule->name : source->entry_name);

[soasme]

Thanks for reporting the issue!