(mALLOc, rEALLOc, cALLOc, mEMALIGn): When

(mALLOc, rEALLOc, cALLOc, mEMALIGn): When
failing to allocate a new chunk, also try the main_arena.  It may
be that we are only running out of mmapped regions, and other
arenas cannot be enlarged sufficiently.
(new_heap, malloc_extend_top): Handle top_pad more consistently,
i.e. only in new_heap().
(fREe): Small optimization.  Before returning,
add fast check whether the heap containing top can go away.