Yes, the Officer would be in possession of the controlled substance, but that is not a crime. It is the "unlawful possession" which is a crime under federal law and so long as it is being held temporarily for turn over to a law enforcement officer it's pretty clearly not "unlawful". And, the same is true for the cop who turns in into evidence and so on.

If your analysis were the case, every cop in America who seized any illegal substance of any kind would be commiting a crime by seizing it.