[9.16] Accept 'in=NULL' with 'inlen=0' in isc_{half}siphash24
requested to merge 3787-siphash-c-105-26-runtime-error-applying-zero-offset-to-null-pointer-v9_16 into v9_16
Arthimetic on NULL pointers is undefined. Avoid arithmetic operations when 'in' is NULL and require 'in' to be non-NULL if 'inlen' is not zero.
(cherry picked from commit 349c23db)
Closes #3787 (closed)
Backport of MR !7339 (merged)