Use safe_to_deref in coregrind syswrap-generic.c (msghdr_foreachfield).

Call ML_(safe_to_deref) before using msghdr msg_name, msg_iov or msg_control.
Fixes bug #334705.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13963 a5019735-40e9-0310-863c-91ae7b9d1cf9