sys-file-reader: Don't warn if compression bias field is 0.

Some software writes a value of 0 to the compression bias field.  We
expect that this field has a value of 100 and warn if we see any other
value, but there is no real need to do so (because it is very likely
that we can read the file correctly in any case).  So suppress the
warning if we see a value of 0 here, because it bothers users.

Thanks to Tony Reardon <tony@sir.com.au> for reporting this
unnecessary warning.