Fix out-of-bounds index check in SimulcastEncoderAdapter::Encode

When frame_types->size() == stream_idx, checking frame_types->size() >= stream_idx causes an out-of-bounds read on (*frame_types)[stream_idx].
This changes the bounds check to frame_types->size() > stream_idx so that shorter frame_types vectors do not cause out-of-bounds indexing.

Bug: b/527798266, webrtc:527515375
Change-Id: I41b971636c565bdb89bbdf7a616a3fe3de69df03
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/493900
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48295}