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
 }

Test/baseResults/150.frag.out

@@ -3,7 +3,9 @@ ERROR: 0:4: 'redeclaration' : cannot redeclare with different qualification: gl_
 ERROR: 0:5: 'redeclaration' : cannot redeclare with different qualification: gl_FragCoord
 ERROR: 0:6: 'layout qualifier' : can only apply origin_upper_left and pixel_center_origin to gl_FragCoord 
 ERROR: 0:14: 'gl_FragCoord' : cannot redeclare after use 
-ERROR: 4 compilation errors.  No code generated.
+ERROR: 0:50: 'gl_PerFragment' : cannot be used (maybe an instance name is needed) 
+ERROR: 0:50: 'gl_PerFragment' : undeclared identifier 
+ERROR: 6 compilation errors.  No code generated.
 
 
 Shader version: 150
@@ -106,6 +108,7 @@ ERROR: node is still EOpNull!
 0:49    Sequence
 0:49      Branch: Return with expression
 0:49        'gl_PrimitiveID' ( flat in int PrimitiveID)
+0:50      'gl_PerFragment' ( temp float)
 0:?   Linker Objects
 0:?     'gl_FragCoord' ( gl_FragCoord 4-component vector of float FragCoord)
 0:?     'foo' ( smooth in 4-component vector of float)