nspi: refactor bind method
nspi: bind method must not free global emsabp_tdb_ctx
[libmapi++] Fix segfault when login() fails.

A failed login calls uninitialize(), but so does the destructor.  A double-free causes a segfault, so always be sure to set freed pointers to 0.
Merge pull request #169 from mactalla/libmapipp-segfault

[libmapi++] Fix segfault when login() fails.
Merge pull request #153 from openchange/jgarcia/nspi_dont_free_global_tdb

nspi: Do not free global tdb