Fix #1879: Check for valid variable before checking for unsized arrays.

The order of error checking was not quite being correct (maybe there is no correct
ordering, when many checks must be done and they affect each other).
So, check for block-name reuse twice.

Test/150.frag

@@ -47,4 +47,5 @@ void barWxyz()
 int primitiveID()
 {
    return gl_PrimitiveID;
+   gl_PerFragment; // ERROR, block name can't get reused
 }