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.

src/decl.c

@@ -957,6 +957,7 @@ const struct const_globals cg = {
     DUMMY, /* zeroobj */
     DUMMY, /* zeromonst */
     DUMMY, /* zeroany */
+    DUMMY, /* zeroNhRect */
 };
 
 #define ZERO(x) memset(&x, 0, sizeof(x))