Fix bug #72185: php-fpm writes empty fcgi record causing nginx 502

This issue might happen if there is change of the fcgi stream when
the buffer is full. Then the empty record is created

Fix bug #72185: php-fpm writes empty fcgi record causing nginx 502

This issue might happen if there is change of the fcgi stream when
the buffer is full. Then the empty record is created which signals
end of stream which is incorrect.

The actual fix without a test was contributed by GitHub user @loveharmful
in GH-3198.

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