ast_function: Fix check for "too few components".

This was triggering even for matrix-from-matrix constructors.  It is
perfectly legal to construct a mat3 from a mat2 - the rest will be
filled in by the identity matrix.

Changes piglit test constructor-23.vert from FAIL to PASS, but the
generated code is incorrect.

src/glsl/ast_function.cpp

@@ -1151,7 +1151,8 @@ ast_function_expression::hir(exec_list *instructions,
        *    arguments to provide an initializer for every component in the
        *    constructed value."
        */
-      if ((components_used < type_components) && (components_used != 1)) {
+      if (components_used < type_components && components_used != 1
+	  && matrix_parameters == 0) {
 	 _mesa_glsl_error(& loc, state, "too few components to construct "
 			  "`%s'",
 			  constructor_type->name);