Fix `warning C4267: '=': conversion from 'size_t' to 'USHORT', possible loss of data`
by explicitly casting to USHORT.
This is OK as the line immediately before NULL-terminates the buffer within boundary.
Addendum to commit 096a69471 (r6279).
'.' and '..' entries do not exist on FATX subdirectories. We have to fake them when
enumerating the directory, and ignore them when creating a FCB from a directory entry.
CORE-16373
