These should never be found in a bzip2 file but it does appear that
there's a buggy encoder that is producing them. Since the official
bzip2 handles this case, this change makes the Go code do likewise.

With this change, the code produces the same output as the official
bzip2 code on the invalid example given in the bug.

Fixes #7279.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/64010043