i965/disasm: Fix INTEL_DEBUG=fs on Broadwell for ARB_fp applications.

Apparently INTEL_DEBUG=fs has crashed on Broadwell for anything using
ARB_fragment_program since commit 9cee3ff5.  We need to NULL-check the
right field.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>

src/mesa/drivers/dri/i965/gen8_fs_generator.cpp

@@ -1255,7 +1255,7 @@ gen8_fs_generator::generate_code(exec_list *instructions)
    int before_size = next_inst_offset - start_offset;
 
    if (unlikely(INTEL_DEBUG & DEBUG_WM)) {
-      if (prog) {
+      if (shader_prog) {
          fprintf(stderr,
                  "Native code for %s fragment shader %d (SIMD%d dispatch):\n",
                 shader_prog->Label ? shader_prog->Label : "unnamed",