Claude: Backport #4729: Upgrade libsrt to 1.5.6 for security enhancement.

Backport the SRT 1.5.6 upgrade from develop (9ff7e24) to SRS 7.0, which
fixes two remotely reachable stack overflows:

- CVE-2026-55869 (upstream PR #3317): processSrtMsg_KMREQ copied bytelen/4
  words into a 104-byte stack buffer with no capacity check.
- CVE-2026-55868 (upstream PR #3319): processSrtMsg_KMRSP did the same with
  no validation of len at all.

The enforced-encryption gate in interpretSrtHandshake blocks the handshake
path when no passphrase is configured, but the post-connect UMSG_EXT control
packet path (CUDT::processSrtMsg) has no such gate and needs no passphrase,
so a peer that completes a handshake could overflow the stack buffer by about
1352 bytes. SRT is off by default at runtime, so only deployments that
explicitly enable srt_server were exposed.

1.5.6 also carries three hardening fixes (upstream PR #3324, #3320, #3322)
for out-of-bounds reads in LOSSREPORT and DROPREQ parsing and a drop-range
guard in CRcvBuffer::dropMessage.

Also backport srs_srt_cleanup(). SRS started libsrt lazily and never stopped
it, so its worker threads outlived the libsrt global objects destroyed at
exit. Since 1.5.6, ~LogDispatcher resets its config pointer to NULL, so a
worker thread logging during teardown dereferences it and crashes. Calling
srt_cleanup() from SrsServer::stop() and from the utest main stops libsrt and
joins those threads first. The utest needs its own call because it has a
separate main that never goes through SrsServer::stop().

This commit carries the code change only. The version bump and the changelog
entry are not included.

Verified on this branch: libsrt builds reporting SRT_VERSION_STRING "1.5.6",
the api.cpp patch applies with no offset, the srs binary links against 1.5.6,
utest passes 2195/2195 over three runs with no SEGV or AddressSanitizer error
under ASAN, and all eight bundled integration scripts pass, including the SRT
publish test verifying RTMP, HTTP-FLV, and HLS playback.

The vendored tree is byte-identical to develop after 9ff7e24.

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: winlinvip

trunk/3rdparty/README.md

@@ -9,8 +9,8 @@ nginx-1.5.7.zip
 * for srs to support hls streaming.
 
 srt-1-fit
-srt-1.5.3.tar.gz
-* https://github.com/Haivision/srt/releases/tag/v1.5.3
+srt-1.5.6.tar.gz
+* https://github.com/Haivision/srt/releases/tag/v1.5.6
 * https://ossrs.net/lts/zh-cn/license#srt
 
 openssl-1.1-fit

trunk/3rdparty/patches/srt/api.cpp-01.patch

@@ -1,4 +1,4 @@
-1116c1116
+1312c1312
 <             LOGC(cnlog.Debug, log << "srt_accept: no pending connection available at the moment");
 ---
 >             LOGC(cnlog.Error, log << "srt_accept: no pending connection available at the moment");