Don't null out the throughput timer, just reset it
It's not necessary to null out the throughput timer and it could be dereferenced if the src is changed while a seek is still "in-flight". Resetting it ensures it is stopped and zeroed and it can be garbage collected along with the rest of the HTTPVideoProvider. This was causing errors in videojs-contrib-hls when the source was changed quickly after a seek was requested.
Merge pull request #125 from videojs/hotfix/throughput-null

Don't null out the throughput timer, just reset it