bpo-34400: Fix more undefined behavior in parsetok.c (GH-8833)

(cherry picked from commit 3e26e42)

Co-authored-by: Zackery Spytz <[email protected]>

Author: miss-islington

Parser/parsetok.c

@@ -252,11 +252,13 @@ parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret,
             }
         }
 #endif
-        if (a >= tok->line_start)
+        if (a != NULL && a >= tok->line_start) {
             col_offset = Py_SAFE_DOWNCAST(a - tok->line_start,
                                           intptr_t, int);
-        else
+        }
+        else {
             col_offset = -1;
+        }
 
         if ((err_ret->error =
              PyParser_AddToken(ps, (int)type, str,