pythongh-71810: Fix corner case (length==0) for int.to_bytes() (python#138739)

```pycon
>>> (0).to_bytes(0, 'big', signed=True)
b''
>>> (-1).to_bytes(0, 'big', signed=True)  # was b''
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    (-1).to_bytes(0, 'big', signed=True)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
OverflowError: int too big to convert
```

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: skirpichev

Lib/test/test_long.py

@@ -1374,17 +1374,22 @@ def equivalent_python(n, length, byteorder, signed=False):
         check(tests4, 'little', signed=False)
 
         self.assertRaises(OverflowError, (256).to_bytes, 1, 'big', signed=False)
-        self.assertRaises(OverflowError, (256).to_bytes, 1, 'big', signed=True)
         self.assertRaises(OverflowError, (256).to_bytes, 1, 'little', signed=False)
-        self.assertRaises(OverflowError, (256).to_bytes, 1, 'little', signed=True)
+        self.assertRaises(OverflowError, (128).to_bytes, 1, 'big', signed=True)
+        self.assertRaises(OverflowError, (128).to_bytes, 1, 'little', signed=True)
+        self.assertRaises(OverflowError, (-129).to_bytes, 1, 'big', signed=True)
+        self.assertRaises(OverflowError, (-129).to_bytes, 1, 'little', signed=True)
         self.assertRaises(OverflowError, (-1).to_bytes, 2, 'big', signed=False)
         self.assertRaises(OverflowError, (-1).to_bytes, 2, 'little', signed=False)
         self.assertEqual((0).to_bytes(0, 'big'), b'')
+        self.assertEqual((0).to_bytes(0, 'big', signed=True), b'')
         self.assertEqual((1).to_bytes(5, 'big'), b'\x00\x00\x00\x00\x01')
         self.assertEqual((0).to_bytes(5, 'big'), b'\x00\x00\x00\x00\x00')
         self.assertEqual((-1).to_bytes(5, 'big', signed=True),
                          b'\xff\xff\xff\xff\xff')
         self.assertRaises(OverflowError, (1).to_bytes, 0, 'big')
+        self.assertRaises(OverflowError, (-1).to_bytes, 0, 'big', signed=True)
+        self.assertRaises(OverflowError, (-1).to_bytes, 0, 'little', signed=True)
 
         # gh-98783
         class SubStr(str):

Misc/NEWS.d/next/Core_and_Builtins/2025-09-10-14-53-59.gh-issue-71810.ppf0J-.rst

@@ -0,0 +1,2 @@
+Raise :exc:`OverflowError` for ``(-1).to_bytes()`` for signed conversions
+when bytes count is zero.  Patch by Sergey B Kirpichev.

Objects/longobject.c

@@ -1209,7 +1209,7 @@ _PyLong_AsByteArray(PyLongObject* v,
         *p = (unsigned char)(accum & 0xff);
         p += pincr;
     }
-    else if (j == n && n > 0 && is_signed) {
+    else if (j == n && is_signed) {
         /* The main loop filled the byte array exactly, so the code
            just above didn't get to ensure there's a sign bit, and the
            loop below wouldn't add one either.  Make sure a sign bit