* Fix error return on sceKernelMunmap
FreeBSD docs state that len <= 0 is a EINVAL return.
* Early return on ReleaseDirectMemory with len = 0
Calls to these two functions with len = 0 cause an assert in CarveDmemArea. Since there's no memory to release, an early return should be safe here.
* Remove check for negative length in munmap
Addresses review comment