Fix likely harmless undefined behavior in cipher tests pointed out by
UBSan with recent compilers (e.g. Clang 10). When the complete output
is empty, the output buffer is NULL. Adding an integer to a null
pointer is undefined behavior even when the integer is 0, so make a
special case for that.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>