slice: don't retry if mmaping was successful and the codec failed
slice: don't retry if mmaping was successful and the codec failed

This prevents squash_splice from attempting to fall back to streaming
if the (de)compression fails: the fallback only occurs if mmapping
fails. In the worst case, this meant (de)compression would be attempted
twice for almost the entirety of the input, if the codec failed on data
at the end of the file.

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