MPRIS extension fixes - #1286

Two commits:

1. Some players occasionally send negative 'pos' values to MPRIS. Clamp these values to 0.
2. The fix for exceptions caused by missing position values in Fix exceptions on mpris-listener extension #1265 causes 'pos' to be set to None, which causes further exceptions. If there's no position (or status) property, do nothing. Refactor the code in question slightly.