st/mesa: Ensure dst in compile_instruction is initialized.

Fixes uninitialized scalar variable defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/state_tracker/st_mesa_to_tgsi.c

@@ -680,7 +680,7 @@ compile_instruction(
 {
    struct ureg_program *ureg = t->ureg;
    GLuint i;
-   struct ureg_dst dst[1];
+   struct ureg_dst dst[1] = { { 0 } };
    struct ureg_src src[4];
    unsigned num_dst;
    unsigned num_src;