Some callers of these functions ask to copy 0 items from a NULL source,
which would be valid if they were copied in a loop (because NULL would
never be dereferenced), but is declared to be undefined behaviour for
Standard C memcpy. Guard the call to memcpy so that we only call it
if we have more than 0 items, and therefore should have a non-NULL
source pointer.

Detected by running a subset of the test suite with
-Dsanitize=address,undefined on x86_64.

Signed-off-by: Simon McVittie <smcv@debian.org>