pulse: compute latency correctly if negative (fixes #15046)
pulse: compute latency correctly if negative (fixes #15046)

This occurs in case of large buffer underflow, such as if stopping the
VLC process.

(cherry picked from commit 887bda11d23665416bd9403151ad1acced68b267)

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2