ralloc: Set *start in ralloc_vasprintf_rewrite_tail() if str is NULL.

We were leaving it undefined, even though we were writing a string to
*str.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/util/ralloc.c

@@ -499,6 +499,7 @@ ralloc_vasprintf_rewrite_tail(char **str, size_t *start, const char *fmt,
    if (unlikely(*str == NULL)) {
       // Assuming a NULL context is probably bad, but it's expected behavior.
       *str = ralloc_vasprintf(NULL, fmt, args);
+      *start = strlen(*str);
       return true;
    }