compress/flate: Fixed two panics on bad data

I used just enough of the data provided by Matt in Issue 5915 to trigger
issue 5915. As luck would have it, using slightly less of it triggered
issue 5962.

Fixes issue 5915.
Fixes issue 5962.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12288043

Committer: Brad Fitzpatrick <bradfitz@golang.org>