USB: serial: garmin_gps: add sanity checking for data length

We must not process packets shorter than a packet ID

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-and-tested-by:  <syzbot+d29e9263e13ce0b9f4fd@syzkaller.appspotmail.com>
Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>