libmthread: resolve memory leaks on exception path

If libmthread runs into a memory allocation failure while attempting
to enlarge its thread pool, it does not free up any preliminary
allocations ma

libmthread: resolve memory leaks on exception path

If libmthread runs into a memory allocation failure while attempting
to enlarge its thread pool, it does not free up any preliminary
allocations made so far.

Reported by dcb314.

This closes #152.

Change-Id: Ib882a4544e4802a0eb0a53446b43997876cde633

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