Fix crashes when dumping stack traces.

In MR_dump_stack_from_layout_clique two variables could be accessed
without being initialised when jumping to the label 'done'.

Also, pair calls to MR_malloc/MR_realloc with MR_free instead of free.
This is purely cosmetic while MR_free is an alias for free.

runtime/mercury_stack_trace.c:
	As above.