Update voice_connection_find() to return the first connection on the line if connection is -1

This and https://dev.iopsys.eu/voice/libvoice-d2/-/merge_requests/17 are for https://project.iopsys.eu/issues/15038.

The input parameter connection to the function voice_connection_find() could be unavailable (-1) sometimes. The first stream on the line shall be returned in this case regardless the actual connection value associated with the stream.