"aalt5_len" in the ATM pseudo-header is not guaranteed to be set to a

"aalt5_len" in the ATM pseudo-header is not guaranteed to be set to a
non-zero value - it's only set from file formats that provide it in a
per-packet header, and only the old DOS Sniffer did so, so it's zero for
all other capture types.  Instead, check the actual packet data length.

Also check it against 16; 14 bytes isn't large enough for a LANE
Ethernet frame.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@16261 f5534014-38df-0310-8fa8-9805f1628bb7