Fix a buffer overflow situation in h2_deliver
Fix a buffer overflow situation in h2_deliver

This still lacks error handling and thus is not a fix for #2589, but at
least we're not writing past the end of the workspace any more.

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