fsck_msdosfs: truncate directory entry when the head pointer is invalid.

Sync with upstream (890cae197737b463e56d1cc5a3f61f84cb49c807).

Bug: 205193362
Fixed: 205193362
Tests: run fsck_msdos on disk image
Change-Id: Idf5a59c2b1951ca51e996b44fa62d72c3d82f81c