In do_palt(), mask out unused data bits.

In do_palt(), mask out unused data bits.

This avoids an infinite loop if one of them is inadvertently (or
advertently) set.  Why might one be set advertently?  Well, it would
allow us to replace our long lists of numbers with short strings.  For
instance, the U+0024 DOLLAR SIGN glyph is "NUTNEUN".  Sadly that would
make the program incompatible with non-ASCII systems and since I claim
ANSI C I should probably continue to support those.