Fix #287: pin down the io-array size before checking for out of range index.

Test/450.geom

@@ -8,7 +8,10 @@ out gl_PerVertex {
     float gl_CullDistance[3];
 };
 
+layout(triangles) in;
+
 void main()
 {
+    gl_in[3].gl_Position; // ERROR, out of range
     gl_CullDistance[2] = gl_in[1].gl_CullDistance[2];
 }