closes bpo-34400: Fix undefined behavior in parsetok(). (GH-4439)

Avoid undefined pointer arithmetic with NULL.
(cherry picked from commit 7c4ab2af)
Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Misc/NEWS.d/next/Core and Builtins/2018-08-14-03-52-43.bpo-34400.AJD0bz.rst

+Fix undefined behavior in parsetok.c.  Patch by Zackery Spytz.

Parser/parsetok.c

@@ -176,7 +176,7 @@ parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret,
         }
         else
             started = 1;
-        len = b - a; /* XXX this may compute NULL - NULL */
+        len = (a != NULL && b != NULL) ? b - a : 0;
         str = (char *) PyObject_MALLOC(len + 1);
         if (str == NULL) {
             fprintf(stderr, "no mem for next token\n");