net/aws: fix sanity check in transcribe loop

When we receive a new alternative we want to avoid iterating out of
bounds, but the comparison between the current index and the length of
the alternative should not log an error when partial_index == length, as
Vec::drain(length..) is valid, and it is completely valid for AWS to
send us a new alternative with as many items as we have already
dequeued.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1751>

net/aws/src/transcriber/transcribe.rs

@@ -237,7 +237,7 @@ impl TranscriberStream {
         mut items: Vec<types::Item>,
         partial: bool,
     ) -> Option<Vec<TranscriptItem>> {
-        if items.len() <= self.partial_index {
+        if items.len() < self.partial_index {
             gst::error!(
                 CAT,
                 imp = self.imp,