Follow on to previous change
Follow on to previous change

If sbrk is shrinking the heap memory size, skip memset (otherwise it would
treat the negative value as a very large positive one and clobber memory).

jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor