Get rid of IndexError in aiohttp web server

[asvetlov]

Fix for #2752

[codecov-io]

Codecov Report

Merging #2763 into 3.0 will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              3.0    #2763      +/-   ##
==========================================
+ Coverage   97.97%   97.97%   +<.01%     
==========================================
  Files          39       39              
  Lines        7340     7341       +1     
  Branches     1285     1286       +1     
==========================================
+ Hits         7191     7192       +1     
  Misses         47       47              
  Partials      102      102

Impacted Files	Coverage Δ
aiohttp/web_protocol.py	91.75% <100%> (+0.02%)	⬆️

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[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new