Panic when using `filter regex` wrong (malformed attribution)

Sorry to keep you busy with these small crashes, but I found another one ;-)

reposurgeon built from current master ( e484d468)

Reproduction:

read <test/simple.fi

# try to replace all commit messages with "foo"
# (but forget 'c' modifier and accidentally replace _all_ attribution fields)
=C filter regex /.*/foo/

# panic: in data traverse of commit: malformed attribution: malformed attribution on 'foo 1287755564 -0400'
#
# goroutine 45 [running]:
# main.(*Repository).dataTraverse.func1(0x1?, {0xa08d50?, 0xc0000e4b40?})
#         /home/florian/projects/3rdparty/reposurgeon/surgeon/inner.go:8943 +0x1255
# main.(*Repository).walkEvents.func1()
#         /home/florian/projects/3rdparty/reposurgeon/surgeon/inner.go:6467 +0xab
# created by main.(*Repository).walkEvents in goroutine 1
#         /home/florian/projects/3rdparty/reposurgeon/surgeon/inner.go:6464 +0xe7
# panic: in data traverse of commit: malformed attribution: malformed attribution on 'foo 1287754582 -0400'

# goroutine 49 [running]:
# main.(*Repository).dataTraverse.func1(0x0?, {0xa08d50?, 0xc0000e4a20?})
#         /home/florian/projects/3rdparty/reposurgeon/surgeon/inner.go:8943 +0x1255
# main.(*Repository).walkEvents.func1()
#         /home/florian/projects/3rdparty/reposurgeon/surgeon/inner.go:6467 +0xab
# created by main.(*Repository).walkEvents in goroutine 1
#         /home/florian/projects/3rdparty/reposurgeon/surgeon/inner.go:6464 +0xe7

Expected: Error message about me doing something stupid, but no crash.