git-blame: prevent argument parsing segfault
git-blame: prevent argument parsing segfault

The 3rd branch in builtin-blame.c should also check for lacking
arguments.  Running that in top dir does not trigger the problem
because the 'prefix' is NULL.

Signed-off-by: Tommi Kyntola <aeebbb8e15f7c5c037a9abaa4c0297165a419b71@ray.fi>
Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git