midx: fix large object offset table check.

It's insufficient to only check if the offset high order bit is set, we
must also check to see if object_large_offsets are in use.

This bug is causing objects to appear missing because they can't be
found in the index.

src/libgit2/midx.c

@@ -431,7 +431,7 @@ int git_midx_entry_find(
 
 	object_offset = idx->object_offsets + pos * 8;
 	offset = ntohl(*((uint32_t *)(object_offset + 4)));
-	if (offset & 0x80000000) {
+	if (idx->object_large_offsets && offset & 0x80000000) {
 		uint32_t object_large_offsets_pos = offset & 0x7fffffff;
 		const unsigned char *object_large_offsets_index = idx->object_large_offsets;