Rework and simplify the code to follow the principle of "dissect as

Rework and simplify the code to follow the principle of "dissect as
 much as possible" rather than dissecting nothing if the length of a
 set of fields (e.g., a "command") seems wrong or if an exception
 occurs fetching data before previous fields have been displayed.
In general: just fetch/dissect from start to end: If there is missing
   data, a Wireshark exception will eventually occur;

Add lwm subtrees under the lwm protocol tree (*not* as protocols
 under the top (outermost) tree.

Fix a bug which caused an exception to be reported as:
 "[Packet size limited during capture: LwMesh truncated]"
 rather than as the correct: "[Malformed...]"

col_...() and expert...() functions shouldn't be called under 'if(tree)';

Register the heuristic dissector using "IEEE802154_PROTOABBREV_WPAN"
 (like certain other dissectors) rather than using "wpan";

missmatch --> mismatch

Fix some long lines.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@54949 f5534014-38df-0310-8fa8-9805f1628bb7