Fix memory leak by freeing qwknode in sbbs_t dtor

I just happened to notice qwknode* in sbbs.h and was like: what is that? Oh, it's a realloced array created when parsing QWK routes. When is it freed? Oh, never.

src/sbbs3/main.cpp

@@ -3810,6 +3810,9 @@ sbbs_t::~sbbs_t()
 	FREE_AND_NULL(usrdirs);
 	FREE_AND_NULL(usrdir);
 
+	FREE_AND_NULL(qwknode);
+	total_qwknodes = 0;
+
 	listFree(&savedlines);
 	listFree(&smb_list);
 	listFree(&mouse_hotspots);