Check backend existence when reading proxy_header
Check backend existence when reading proxy_header

A probe may still be running when its backend is deleted. In that case
it needs to read the proxy_header field under the lock and give up if
the backend was already gone.

Not sure how to write a test case for this one.

Fixes #2278

feld/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache