md5: Consistently use item count as the first argument to calloc
Reported by: GCC 14 -Wcalloc-transposed-args
Reviewed by: rlibby
Differential Revision: https://reviews.freebsd.org/D46009
(cherry picked from commit e8e2ab3c55228d66388a0b33de8c5ac4b3d6dbe9)