FreeBSD: Do not try to cast NULL to int.

[rakuco]

Passing NULL to the data parameter in calls to ptrace(2) causes failures on 64-bit FreeBSD 9 installations, where GCC 4.2.1 complains like this:

platform/freebsd/arch/x86_common.h: In function 'arch_get_register':
platform/freebsd/arch/x86_common.h:48: warning: cast from pointer to integer of different size
platform/freebsd/arch/x86_common.h: In function 'arch_set_register':
platform/freebsd/arch/x86_common.h:57: warning: cast from pointer to integer of different size
platform/freebsd/arch/x86_common.h:59: warning: cast from pointer to integer of different size
platform/freebsd/freebsd_ptrace.c: In function 'ptrace_memcpy_to_child':
platform/freebsd/freebsd_ptrace.c:267: warning: cast from pointer to integer of different size
platform/freebsd/freebsd_ptrace.c: In function 'ptrace_memcpy_from_child':
platform/freebsd/freebsd_ptrace.c:282: warning: cast from pointer to integer of different size

Make ptrace_command pass 0 to _ptrace_command so that the final expansion looks like this:

__ptrace_command((cld), (req), (void*)(addr), (int)(0))

instead of

__ptrace_command((cld), (req), (void*)(addr), (int)(((void*)0)))

[nelhage]

:+1: thanks