revid: rv panics when no Input variable is specified but InputCodec is set to PCM

rv exits with a panic when no Input variable is specified but InputCodec is set to PCM on vidgrind.

panic: unknown input codec for raspivid input

Either it should not be possible for the Input and InputCodec to be an invalid match, in which case we should keep the panic but find where the mismatch is happening. Or we should not panic, just throw an error since it’s possible for a user to cause this error.