Backport of "srt: Fix "Fix timestamping"" into 1.18

Backport of !1674 (merged) "srt: Fix "Fix timestamping"".

Commits:

• 6b2fcb52 srtsrc: Defend against missing clock
• ec11ad9d srtsrc: Don't calculate a delay if the srctime is 0
• 4eeff95f srtsrc: Prevent delay from being negative