[patch] Fix ICEs when DEBUG_MANGLE is enabled

[Cary Coutant]

> 2013-11-13  Cary Coutant  <ccoutant@google.com>
>
> gcc/cp/
>         * mangle.c (to_base36): New function.
>         (dump_substitution_candidates): Add checks for NULL.
>         Print substitution index in base 36.

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-cary