Fix bug 7045 - Bad (non memory copying) interfaces in smbc_setXXXX calls.

Fix bug 7045 - Bad (non memory copying) interfaces in smbc_setXXXX calls.

In smbc_free_context libsmbclient just called free() on the string options
so it assumes the callers have malloced them before setting them via smbc_set
calls.

Change to correctly malloc/free string options to the library.
Protect against SMB_STRDUP of null.

Contains 2d41b1ab78639abe4ae030ff482573f464564dd7 and
f85b6ee90b88c7f7b2a92c8a5f3e2ebe59c1087b from master.

Jeremy
(cherry picked from commit edc44312f76e14e94c56e70cf7bb49139f9f081e)