gallivm: fix default cbuf info.

The default null_output really needs to be static, otherwise the values
we'll eventually get later are doubly random (they are not initialized,
and even if they were it's a pointer to a local stack variable).
VMware bug 2349556.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/gallivm/lp_bld_tgsi_info.c

@@ -608,7 +608,7 @@ finished:
     */
 
    for (index = 0; index < PIPE_MAX_COLOR_BUFS; ++index) {
-      const struct lp_tgsi_channel_info null_output[4];
+      static const struct lp_tgsi_channel_info null_output[4];
       info->cbuf[index] = null_output;
    }