dnsdist: Fix a null-deref in incoming DoH w/ nghttp2

dnsdist: Fix a null-deref in incoming DoH w/ nghttp2

When an incoming DoH connection using the `nghttp2` provider is waiting
for a response from a backend that results in a I/O error or timeout,
and the incoming connection also fails due to a I/O error or timeout,
dnsdist could in some cases try to dereference a null pointer, leading
to a crash.