rtpjitterbuffer: Fix parsing of the mediaclk:direct= field

Due to an off-by-one when parsing the string, the most significant digit
or the clock offset was skipped when parsing the offset.

Part-of: <gstreamer/gst-plugins-good!910>
32 jobs for !910 with backport-mr-907-into-1.18-mediaclk-direct-offset-parsing in 31 minutes and 2 seconds (queued for 20 seconds)
merge request