Fix bug #7698 - Assert causes smbd to panic on invalid NetBIOS session request.

Fix bug #7698 - Assert causes smbd to panic on invalid NetBIOS session request.

Found by the CodeNomicon test suites at the SNIA plugfest.

http://www.codenomicon.com/

If an invalid NetBIOS session request is received the code in name_len() in
libsmb/nmblib.c can hit an assert.

Re-write name_len() and name_extract() to use "buf/len" pairs and
always limit reads.

Jeremy.