RED5DEV-2156 playengine: clamp negative late-subscriber adjusted timestamps

When PlayEngine pushes a buffered keyframe (or any prelude event)
captured before the live anchor (videoBaseTs / audioBaseTs / publisher
timestamp offset), the rebase formula eventTime - baseTs + 2 produces
a negative result. The negative int writes to the wire as the equivalent
unsigned-32 value, which RTMP clients render as a far-future timestamp
(e.g. -97 ms -> 0xFFFFFF9F -> ~24.85 days when bit 31 is stripped, or
~49.7 days when read as full unsigned-32).

Clamp negative adjusted timestamps to 1 so buffered events precede the
first live frame at ts=2 without leaking into the unsigned-32 upper
half. Verified via ffplay end-to-end against an SRT/TS publisher: video
start went from 2147483.551s to 0.001s.

Author: mondain

common/src/main/java/org/red5/server/stream/PlayEngine.java

@@ -1199,6 +1199,16 @@ private void sendMessage(RTMPMessage messageIn) {
                         eventTime -= startTs;
                     }
                 }
+                // Clamp negative adjusted timestamps. Buffered events (decoder configs, keyframes)
+                // captured before the live anchor land here with eventTime < 0; place them at ts=1
+                // so they precede the first live frame at ts=2 without being interpreted as a
+                // far-future unsigned-32 value (e.g. -97 on the wire = 0xFFFFFF9F = ~49.7 days).
+                if (eventTime < 0) {
+                    if (isTrace) {
+                        log.trace("sendMessage: clamping negative adjusted timestamp {} to 1 (buffered prelude before live anchor)", eventTime);
+                    }
+                    eventTime = 1;
+                }
                 messageOut.getBody().setTimestamp(eventTime);
                 if (isTrace) {
                     log.trace("sendMessage (updated): streamStartTS={}, length={}, streamOffset={}, timestamp={}", new Object[] { startTs, currentItem.get().getLength(), streamOffset, eventTime });