glsl: Initialize variable in ir_swizzle::constant_expression_value.

Complete initialize data passed to ir_constant constructor.

Fixes piglit glsl-mat-from-int-ctor-02 valgrind unintialized variable
error with softpipe and llvmpipe.

src/glsl/ir_constant_expression.cpp

@@ -653,7 +653,7 @@ ir_swizzle::constant_expression_value()
    ir_constant *v = this->val->constant_expression_value();
 
    if (v != NULL) {
-      ir_constant_data data;
+      ir_constant_data data = { { 0 } };
 
       const unsigned swiz_idx[4] = {
 	 this->mask.x, this->mask.y, this->mask.z, this->mask.w