bugfix: fix zstd module infinite loop when upstream return content-length abnormal

[drawing]

When the backend business returns a content-length exception, which the content-length is greater than the actual body size(for example, restart when the business return data is not completed), nginx will send a flush empty buffer, which will cause an infinite loop of zstd filter module.

for example, upstream return content:

HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Content-Length: 2

a

[ThinkChaos]

Ran into this bug and can confirm this fix worked!

Would be nice to get it merged to save time for others :)

[melroy89]

Somebody merge it?