Fix a crash when properties have setters but not getters.

sourcepawn/compiler/tests/ok-setter-no-getter.sp

@@ -0,0 +1,12 @@
+methodmap Crab
+{
+	property int X {
+		public native set(int value);
+	}
+}
+
+public main()
+{
+	Crab crab;
+	crab.X = 12;
+}