fat16copy: Fix allocation logic when extending directories.

When extending a directory, empty directory entries were left behind
signaling the end of a directory.

Bug: 31570546
Test: make dist; mdir shows all the files.

(cherry picked from commit d4516300b010c8155861e310dac871e8c10d3382)

Change-Id: Ie3d82cfaad51be73911293a54d18746e8adf62a2