[prutschman]

In the context of the code presented all mallocs were returned by sbrk. Even if the previous malloc had been "allocated" with size zero, the sbrk called by this function's invocation of malloc will ensure that there are enough bytes of addressable memory to ensure that, although it will be copying data out of earlier allocations, it will not segfault. realloc makes no guarantees of zero-initializing a newly allocated extent.