Don't copy memory for arguments if there are none

Don't copy memory for arguments if there are none

If amd64_windows_push_arguments is called with no arguments, then ARGS
can be NULL, and inside the passed-by-pointer block, memcpy is called
with this NULL, which is undefined behavior.

So this just disable the passed-by-pointer block if there are no
arguments.

Fixes the following ubsan error:
C:/gdb/src/gdb.git/gdb/amd64-windows-tdep.c:244:12: runtime error: null pointer passed as argument 2, which is declared to never be null

Approved-By: Tom Tromey <tom@tromey.com>