protocol/RangeArg: add Contains()

src/db/DatabasePrint.cxx

@@ -202,9 +202,7 @@ db_selection_print(Response &r, Partition &partition,
 	if (sort == TAG_NUM_OF_ITEM_TYPES) {
 		if (!window.IsAll())
 			s = [s, window, &i](const LightSong &song){
-				const bool in_window = i >= window.start && i < window.end;
-				++i;
-				if (in_window)
+				if (window.Contains(i++))
 					s(song);
 			};
 

src/protocol/RangeArg.hxx

@@ -42,6 +42,10 @@ struct RangeArg {
 	constexpr bool IsAll() const noexcept {
 		return *this == All();
 	}
+
+	constexpr bool Contains(unsigned i) const noexcept {
+		return i >= start && i < end;
+	}
 };
 
 #endif