Avoid undefined memcpy/memmove invocations

Avoid undefined memcpy/memmove invocations

Where memcpy() or memmove() might be invoked with null input or output
pointers, add checks for non-zero length.  For brevity, add helpers
k5memcpy() and k5memmove() and use them in files that include
k5-int.h.

Most of the potential undefined invocations were reported by Evgeny
Shemyakin.