streams: bivalent stream signals a condition for bytes out of range

Sometimes a byte may be not within the character code range. In that case, when
we read the char, the system will signal a condition.

Alternatively (and that's the behavior before this commit) we could return the
character #\Nul. That was done by virtue of ECL_CHAR_CODE skipping tag bytes, so
the returned NIL was treated as 0.