radv: only reset command buffers when the allocation fails

radv: only reset command buffers when the allocation fails

   "vkAllocateCommandBuffers can be used to create multiple command
    buffers. If the creation of any of those command buffers fails, the
    implementation must destroy all successfully created command buffer
    objects from this command, set all entries of the pCommandBuffers
    array to NULL and return the error."

This has been suggested by gabriel@system.is.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>