update-index: fix segfault with missing --cacheinfo argument

Running "git update-index --cacheinfo" without any further
arguments results in a segfault rather than an error
message. Commit ec160ae (update-index: teach --cacheinfo a
new syntax "mode,sha1,path", 2014-03-23) added code to
examine the format of the argument, but forgot to handle the
NULL case.

Returning an error from the parser is enough, since we then
treat it as an old-style "--cacheinfo <mode> <sha1> <path>",
and complain that we have less than 3 arguments to read.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>