Issue #15989: Fix several occurrences of integer overflow
when result of PyLong_AsLong() narrowed to int without checks.

This is a backport of changesets 13e2e44db99d and 525407d89277.
diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 413f9dd..6792179 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -5,6 +5,7 @@
 import unittest, string, sys, struct
 from test import support
 from collections import UserList
+import _testcapi
 
 class Sequence:
     def __init__(self, seq='wxyz'): self.seq = seq
@@ -1142,6 +1143,16 @@
         self.checkraises(TypeError, '%10.*f', '__mod__', ('foo', 42.))
         self.checkraises(ValueError, '%10', '__mod__', (42,))
 
+        self.checkraises(OverflowError, '%*s', '__mod__',
+                         (_testcapi.PY_SSIZE_T_MAX + 1, ''))
+        self.checkraises(OverflowError, '%.*f', '__mod__',
+                         (_testcapi.INT_MAX + 1, 1. / 7))
+        # Issue 15989
+        self.checkraises(OverflowError, '%*s', '__mod__',
+                         (1 << (_testcapi.PY_SSIZE_T_MAX.bit_length() + 1), ''))
+        self.checkraises(OverflowError, '%.*f', '__mod__',
+                         (_testcapi.UINT_MAX + 1, 1. / 7))
+
         class X(object): pass
         self.checkraises(TypeError, 'abc', '__mod__', X())