fsck: avoid looking at NULL blob->object

Commit 159e7b080b (fsck: detect gitmodules files, 2018-05-02) taught fsck to look at the content of .gitmodules files. If the object turns out not to be a blob at all, we just complain and punt on checking the content. And since this was such an obvious and trivial code path, I didn't even bother to add a test. Except it _does_ do one non-trivial thing, which is call the report() function, which wants us to pass a pointer to a "struct object". Which we don't have (we have only a "struct object_id"). So we erroneously pass a NULL object to report(), which gets dereferenced and causes a segfault. It seems like we could refactor report() to just take the object_id itself. But we pass the object pointer along to a callback function, and indeed this ends up in builtin/fsck.c's objreport() which does want to look at other parts of the object (like the type). So instead, let's just use lookup_unknown_object() to get the real "struct object", and pass that. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>