[NETFILTER]: H.323 helper: fix endless loop caused by invalid TPKT len

When the TPKT len included in the packet is below the lowest valid value
of 4 an underflow occurs which results in an endless loop.

Found by testcase 0000058 from the PROTOS c07-h2250v4 testsuite.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>