HTC_RxInit and HTC_RxReInit could write a single '\0' NUL character
outside of the workspace when its called and there is zero bytes left
in the workspace. This would trigger the workspace canary causing
subsequent assertion.

Fix by releaving HTC_RxInit and HTC_RxReInit of adding the '\0'
character.

HTC_RxStuff and V1F_FetchRespHdr returns HTC_S_OVERFLOW if the
available buffer space is zero. Both make sure to insert the '\0'
character just before calling the completion check function.

Note that this fix does not change the fact that we have exchausted
the workspace and are unable to continue. Varnishd will panic
nonetheless, but at least we have not stepped out of our boundries.

Ref: #1834