init NhRect, before passing to selection_getbounds

selection_getbounds() has a check and early return.
Initialization will ensure a known state if that early return
were ever taken.

This is an alternative approach to pr #1163.

include/decl.h

@@ -1150,6 +1150,7 @@ struct const_globals {
     const struct obj zeroobj;      /* used to zero out a struct obj */
     const struct monst zeromonst;  /* used to zero out a struct monst */
     const anything zeroany;        /* used to zero out union any */
+    const NhRect zeroNhRect;       /* used to zero out NhRect */
 };
 
 extern const struct const_globals cg;