Fix the error of not accumulating nwrite in srs_write_large_iovs

[lam2003]

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TRANS_BY_GPT3

[codecov-io]

Codecov Report

Merging #1651 into 3.0release will decrease coverage by 0.03%.
The diff coverage is 75%.

@@              Coverage Diff               @@
##           3.0release    #1651      +/-   ##
==============================================
- Coverage       52.89%   52.86%   -0.04%     
==============================================
  Files              82       82              
  Lines           26619    26622       +3     
==============================================
- Hits            14080    14073       -7     
- Misses          12539    12549      +10

| Impacted Files | Coverage Δ | |'

Translated to English while maintaining the markdown structure:

'| Impacted Files | Coverage Δ | |
|---|---|---|
| trunk/src/protocol/srs_protocol_utility.cpp | 96.04% <75%> (-0.51%) | ⬇️ |
| trunk/src/core/srs_core_autofree.hpp | 0% <0%> (-100%) | ⬇️ |

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TRANS_BY_GPT3

[winlinvip]

Add utest to cover code in f89b4b3