handle mremap failure by falling back to memcpy
handle mremap failure by falling back to memcpy

The previous code also leaked the allocated chunks because it returned
without calling chunk_free. In practice, this code path is unused as the
mremap call will only fail when splitting the source VMA fails. Ideally,
it wouldn't unmap the source and failure would be even rarer.

thestinger/allocator,thestinger/allocator