Backport #4729: Upgrade libsrt to 1.5.6 for security enhancement. v7.0.160 (#4730)

Backport of #4729 to SRS 7.0: upgrade the vendored SRT library from
1.5.3 to 1.5.6 to fix two remotely reachable security vulnerabilities.

Backports #4729. Relates to #4727.

SRS 7.0 vendors SRT 1.5.3 and is affected by both CVEs exactly as
`develop` was:

- **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 it does **not**
cover the post-connect `UMSG_EXT` control packet path
(`CUDT::processSrtMsg`), which has no passphrase gate. At the default
MSS of 1500 that is roughly **1352 bytes of stack overflow** from any
peer that completes a handshake, before SRS-level streamid or callback
auth ever runs.

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 PRs #3324, #3320,
#3322) for out-of-bounds reads in LOSSREPORT and DROPREQ parsing and a
drop-range guard in `CRcvBuffer::dropMessage`. These matter less in SRS
— the two OOB reads land inside the allocated packet slot, and the
`dropMessage` guard is unreachable under SRS defaults.

Full reachability analysis:
#4727 (comment)

---

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");