sys-file-reader: Tolerate nonsensical opcodes in compressed data.

Compressed data in .sav files uses a set of 256 opcodes, some of which make
sense only for numeric data and others of which only make sense for string
data.  However, Jereme Thomas <jereme.thomas@gmail.com> has provided one
file, written by SPSS 14, that uses an opcode that seems to makes sense
only for numeric data in a string field.  So this commit adds support for
these opcodes, although it still warns about the ones other than the exact
one found in the file provided by Jereme.