i965: Don't try to unmap NULL program cache BO.

When running shader-db with intel_stub and recent Mesa, context creation
fails when making a logical hardware context.  In this case, we call
intelDestroyContext(), which gets here and tries to unmap the cache BO.

But there isn't one - we haven't made it yet.  So we try to unmap a
NULL pointer, which used to be safe (it did nothing), but crashes
after commit 7c3b8ed878.

The result is that we crash rather than failing context creation with
a nice message.  Either way nothing works, but this is more polite.

Cc: "17.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>

src/mesa/drivers/dri/i965/brw_program_cache.c

@@ -485,10 +485,13 @@ brw_destroy_cache(struct brw_context *brw, struct brw_cache *cache)
 
    DBG("%s\n", __func__);
 
-   if (brw->has_llc)
-      brw_bo_unmap(cache->bo);
-   brw_bo_unreference(cache->bo);
-   cache->bo = NULL;
+   /* This can be NULL if context creation failed early on */
+   if (cache->bo) {
+      if (brw->has_llc)
+         brw_bo_unmap(cache->bo);
+      brw_bo_unreference(cache->bo);
+      cache->bo = NULL;
+   }
    brw_clear_cache(brw, cache);
    free(cache->items);
    cache->items = NULL;