lzf: work around bug when compressing a single byte
lzf: work around bug when compressing a single byte

The issue has been reported, and once it is fixed in LZF I'll revert
this.  In the meantime, this should make the reports from
AddressSanitizer clean when using LZF.

https://github.com/quixdb/squash/issues/129

jibsen/squash,Cloudef/squash,Cloudef/squash,quixdb/squash,quixdb/squash,jibsen/squash,Cloudef/squash,quixdb/squash,jibsen/squash