dvb-core: Proper handling ULE SNDU length of 0 (CVE-2006-4623)

ULE (Unidirectional Lightweight Encapsulation RFC 4326) decapsulation
code has a bug that allows an attacker to send a malformed ULE packet
with SNDU length of 0 and bring down the receiving machine. This patch
fix the bug and has been tested on version 2.6.17.11. This bug is 100%
reproducible and the modified source code (GPL) used to produce this bug
will be posted on http://nrg.cs.usm.my/downloads.htm

 shortly.  The
kernel will produce a dump during CRC32 checking on faulty ULE packet.

Signed-off-by: Ang Way Chuang <wcang@nrg.cs.usm.my>
Signed-off-by: Adrian Bunk <bunk@stusta.de>