fsck: avoid using an uninitialized variable

In `fsck_commit()`, after counting the authors of a commit, we set the
`err` variable either when there was no author, or when there were more
than two authors recorded. Then we access the `err` variable to figure
out whether we should return early. But if there was exactly one author,
that variable is still uninitialized.

Let's just initialize the variable.

This issue was pointed out by CodeQL.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

fsck.c

@@ -926,7 +926,7 @@ static int fsck_commit(const struct object_id *oid,
 {
 	struct object_id tree_oid, parent_oid;
 	unsigned author_count;
-	int err;
+	int err = 0;
 	const char *buffer_begin = buffer;
 	const char *buffer_end = buffer + size;
 	const char *p;