slcanpty: handle incomplete messages from pty correctly

slcanpty: handle incomplete messages from pty correctly

Make sure that we have minimum one complete SLCAN messages from pty in the
receive buffer before we start processing the received SLCAN message.

Fragments of a received incomplete message are stored to be appended by the
next read() syscall in pty2can().

This patch is a rework of an initial patch from Ulrich Escher. Tnx!

Reported-by: Ulrich Escher <git@myvdr.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>