do_coredump(): check return from argv_split()

do_coredump() accesses helper_argv[0] without checking helper_argv !=
NULL.  This can happen if page allocation failed.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>