* Detect a long header line which doesn't fit the buffer.
* If the header line in question doesn't seem important for us,
just ignore it.
* Otherwise, bail out with -E2BIG.
The overrun was found by LLVM UBSan on macOS.
The following is an example of a long header line,
which doesn't fit the default CONFIG_WEBCLIENT_MAXHTTPLINE=200.
```
pacetanuki% curl -v https://www.midokura.com
:
:
< HTTP/2 200
< server: nginx
< date: Fri, 14 May 2021 02:16:24 GMT
< content-type: text/html; charset=UTF-8
< content-length: 131313
< x-powered-by: PHP/7.4.18
< link: <https://www.midokura.com/wp-json/>; rel="https://api.w.org/", <https://www.midokura.com/wp-json/wp/v2/pages/7>; rel="alternate"; type="application/json", <https://www.midokura.com/>; rel=shortlink
< x-powered-by: PleskLin
<
```
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