The gunzip vdp failed to handle junk after end of gzip data. This
basically mirrors #942 on the client side, also the fix is basically the
same as 41f7a356

The impact of this bug is likely to be low, because the built-in
beresp.filters logic will push the testgunzip VFP for gzip content
received from backends, so, unless VCL is forced to pass backend
responses unchecked or vmods generate body data, it can be considered
unlikely that this issue will be hit.

Fixes #3109