is_ntfs_dotgit: use a size_t for traversing string

We walk through the "name" string using an int, which can wrap to a negative value and cause us to read random memory before our array (e.g., by creating a tree with a name >2GB, since "int" is still 32 bits even on most 64-bit platforms). Worse, this is easy to trigger during the fsck_tree() check, which is supposed to be protecting us from malicious garbage. Note one bit of trickiness in the existing code: we sometimes assign -1 to "len" at the end of the loop, and then rely on the "len++" in the for-loop's increment to take it back to 0. This is still legal with a size_t, since assigning -1 will turn into SIZE_MAX, which then wraps around to 0 on increment. Signed-off-by: Jeff King <peff@peff.net>