aws_transcriber: sanity check alternative length

The design of the element is based on the assumption that when receiving a partial result, the following result will contain at least as many items as there were stable items in the previous result.

This patch adds a sanity check to make sure our "partial index" isn't larger than the new received result, and errors out otherwise.

partial_index will eventually be reset to 0 once we receive a new non-partial result.