Don't free() NULL pointers

Don't free() NULL pointers

The alloc() function in the test suite unconditionally succeeds when
trying to allocate zero bytes.  It does so by returning NULL right away,
without exiting the program.  This was for portability for platforms
that refuse to allocate zero bytes.

Unfortunately, this meant that the test suite later called free() on
those NULL pointers, which is undefined.  Wrapping free() in a dealloc()
function avoids this error.