Ext4Pkg: Fix out-of-bounds read in Ext4ReadDir

Fix an out-of-bounds read inside CompareMem() when
checking for "." or ".." by explicitly bounding name_len
to [0, 2] beforehand.

Reported-by: Savva Mitrofanov <savvamtr@gmail.com>
Fixes: 45e37d85 ("Ext4Pkg: Hide "." and ".." entries from Read() callers.")
Cc: Marvin Häuser <mhaeuser@posteo.de>
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>