default_file_op: read: Fix bug causing SEGV
default_file_op: read: Fix bug causing SEGV

 In default_file_op::read(), if `offset` value is
larger than file size, then out of bounds of read is occurred.
 Fix this by returning 0 if `offset` is larger than file size.

pfpacket/fuse_planetfs,pfpacket/fuse_planetfs,pfpacket/fuse_planetfs,pfpacket/fuse_planetfs