glsl: Initialize builtin_builder member variables.

Fixes "Uninitialized pointer field" defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/builtin_functions.cpp

@@ -525,6 +525,9 @@ private:
  *  @{
  */
 builtin_builder::builtin_builder()
+   : shader(NULL),
+     gl_ModelViewProjectionMatrix(NULL),
+     gl_Vertex(NULL)
 {
    mem_ctx = NULL;
 }