[3.10] gh-91700: Validate the group number in conditional expression in RE (GH-91702) (GH-91831)

In expression (?(group)...) an appropriate re.error is now
raised if the group number refers to not defined group.

Previously it raised RuntimeError: invalid SRE code.
(cherry picked from commit 48ec61a89a)

Lib/test/test_re.py

@@ -575,6 +575,8 @@ def test_re_groupref_exists_errors(self):
         self.checkPatternError(r'()(?(1)a|b|c)',
                                'conditional backref with more than '
                                'two branches', 10)
+        self.checkPatternError(r'()(?(2)a)',
+                               "invalid group reference 2", 5)
 
     def test_re_groupref_overflow(self):
         from sre_constants import MAXGROUPS