i965/vs: Fix uninitialized shader pointer used in debug output.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_vec4_emit.cpp

@@ -141,6 +141,8 @@ vec4_generator::vec4_generator(struct brw_context *brw,
    intel = &brw->intel;
    vp = &c->vp->program;
 
+   shader = prog ? prog->_LinkedShaders[MESA_SHADER_VERTEX] : NULL;
+
    p = rzalloc(mem_ctx, struct brw_compile);
    brw_init_compile(brw, p, mem_ctx);
 }