Fix segfault in case of the database lock not being available.

We were nicely reporting the lock-aquisition failure, but then marching
along trying to use the database object and just crashing badly.
So don't do that.

database.cc

@@ -489,6 +489,7 @@ notmuch_database_open (const char *path)
     } catch (const Xapian::Error &error) {
 	fprintf (stderr, "A Xapian exception occurred: %s\n",
 		 error.get_msg().c_str());
+	notmuch = NULL;
     }
     
   DONE: