Fix bug in dlmalloc's version of calloc.

Under some circumstances, doing a calloc will make sure that the memory
returned will be zero up to the size of the requested size. However, if
there is more usable size than the requested size, that extra part
of the allocation will not be zeroed. This change fixes it so that the
entire usable memory is always zeroed.

Change-Id: I8a66d6767c074023c4ba3568bf2705e1886740fc