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 #5915.
Fixes #5962.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12288043
2 files changed