[MP] Fixed a hang in S_UpdateBackgroundTrack_Actual() when using some sample rates.

Refer to a similar commit to ioq3: https://github.com/ioquake/ioq3/commit/b9f033d1fa2a5f339654c48bcb22201d72a3de44

This resolves a hang seen when using mono 22 kHz music files with s_khz set to 44, and potentially other cases.