core/alloc.c, core/allocator.c: fix calloc(3) and free(3) usage

core/alloc.c, core/allocator.c: fix calloc(3) and free(3) usage

calloc(3)'s 2 arguments are, in order: number of members, size of member.
-> Swap the arguments to the correct order.

free(3) is well-documented to do nothing when passed NULL.
-> Remove conditional check for non-NULL.

Also add malloc and alloc_size function attributes where necessary.