bpo-45820: Fix a segfault when the parser fails without reading any input (GH-29580)

Author: pablogsal

Misc/NEWS.d/next/Core and Builtins/2021-11-16-19-00-27.bpo-45820.2X6Psr.rst

@@ -0,0 +1,2 @@
+Fix a segfault when the parser fails without reading any input. Patch by
+Pablo Galindo

Parser/pegen.c

@@ -368,6 +368,14 @@ tokenizer_error(Parser *p)
 void *
 _PyPegen_raise_error(Parser *p, PyObject *errtype, const char *errmsg, ...)
 {
+    if (p->fill == 0) {
+        va_list va;
+        va_start(va, errmsg);
+        _PyPegen_raise_error_known_location(p, errtype, 0, 0, 0, -1, errmsg, va);
+        va_end(va);
+        return NULL;
+    }
+
     Token *t = p->known_err_token != NULL ? p->known_err_token : p->tokens[p->fill - 1];
     Py_ssize_t col_offset;
     Py_ssize_t end_col_offset = -1;