tracing/kprobe: Fix NULL pointer deref check

Add NULL check for avoiding NULL pointer deref.
This bug has been introduced by:

  1ff511e3: tracing/kprobes: Add bitfield type

which causes a null pointer dereference bug when kprobe-tracer
parses an argument without type.

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: 2nddept-manager@sdl.hitachi.co.jp
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20110214054807.8919.69740.stgit@ltc236.sdl.hitachi.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Reported-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>