clean-up
MemPtr.xs: use newSVpvn() instead of newSVpm() with explicit STRLEN

FFI::Raw::MemPtr::tostr() should use newSVpvn(), not newSVpv(), when a length
argument is supplied. Otherwise a length argument of 0 is treated by newSVpv()
as an instruction to use strlen() to find the length, not as a length of 0.
Changes: update