Try to fix#4450
## Cause
The SRS transcode rtp packets, whose sequence number in range [start,
end], to one rtmp packet, but when the first rtp packet is empty, then
this crash happens.
check #4450 for details.
## Impact
5.0release and 6.0release branch.
develop branch already has its own solution.
So this PR is targeting to **6.0release**.
## Solution
find the first not empty rtp packet in seq range [start, end].
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Co-authored-by: OSSRS-AI <winlinam@gmail.com>
Co-authored-by: winlin <winlinvip@gmail.com>
c1a8a5f753