fs/squashfs: fix board hang-up when calling .exists()

add missing squashfs function to prevent dangling or null pointers.
For exemple, when calling test [ -e somefile ], squashfs.exists may be
called.

Signed-off-by: Richard Genoud <richard.genoud@posteo.net>
Reviewed-by: Joao Marcos Costa <jmcosta944@gmail.com>
1 file changed