gh-98740: Fix validation of conditional expressions in RE (GH-98764)

In very rare circumstances the JUMP opcode could be confused with the
argument of the opcode in the "then" part which doesn't end with the
JUMP opcode. This led to incorrect detection of the final JUMP opcode
and incorrect calculation of the size of the subexpression.

NOTE: Changed return value of functions _validate_inner() and
_validate_charset() in Modules/_sre/sre.c.  Now they return 0 on success,
-1 on failure, and 1 if the last op is JUMP (which usually is a failure).
Previously they returned 1 on success and 0 on failure.

Doc/library/re.rst

@@ -483,6 +483,9 @@ The special characters are:
    some fixed length.  Patterns which start with negative lookbehind assertions may
    match at the beginning of the string being searched.
 
+.. _re-conditional-expression:
+.. index:: single: (?(; in regular expressions
+
 ``(?(id/name)yes-pattern|no-pattern)``
    Will try to match with ``yes-pattern`` if the group with given *id* or
    *name* exists, and with ``no-pattern`` if it doesn't. ``no-pattern`` is