added some defensive programming to nmbd. This mostly means zeroing

added some defensive programming to nmbd. This mostly means zeroing
areas of memory before freeing them.

While doing this I also found a couple of real bugs. In two places we
were freeing some memory that came from the stack, which leads to
a certain core dump on many sytems.
(This used to be commit c5e5c25c854e54f59291057ba47c4701b5910ebe)