vulkan/overlay: fix crash on freeing NULL command buffer

It is legal to call vkFreeCommandBuffers() on NULL command buffers.

This fix requires eb41ce1b ("util/hash_table: Properly handle
the NULL key in hash_table_u64").

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 4438188f ("vulkan/overlay: record stats in command buffers and accumulate on exec/submit")
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit a72351cc)