fs/proc/vmcore.c: remove NULL assignment to static

Static values are automatically initialized to NULL.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>