shallow: fix NULL dereference

After `write_one_shallow()` calls `lookup_commit()` to find the commit
object for a shallow graft entry, it then checks `if (!c || ...)`.
Inside that block, when the VERBOSE flag is set, it prints the OID being
removed, via `c->object.oid`. But `c` can be NULL (the first condition
in the `||` check).

This happens when a shallow graft entry references a commit object that
is not in the object store (e.g., after a partial fetch or in a
corrupted repository). In that case, `lookup_commit()` returns NULL
because the object cannot be found, the SEEN_ONLY check correctly
decides to remove this entry from .git/shallow, but the verbose message
crashes before the removal can complete.

Use `graft->oid` instead of `c->object.oid` for the message. The graft
entry's OID is the same value (it was used as the lookup key) and is
always available regardless of whether the commit object exists.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

shallow.c

@@ -371,7 +371,7 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 		if (!c || !(c->object.flags & SEEN)) {
 			if (data->flags & VERBOSE)
 				printf("Removing %s from .git/shallow\n",
-				       oid_to_hex(&c->object.oid));
+				       oid_to_hex(&graft->oid));
 			return 0;
 		}
 	}