bpo-35194: Fix a wrong constant in cp932 codec (GH-10420)

This typo doesn't affect the result because wrong bits are discarded
on implicit conversion to unsigned char, but it trips UBSan
with -fsanitize=implicit-integer-truncation.

https://bugs.python.org/issue35194
(cherry picked from commit 7a69cf47)
Co-authored-by: Alexey Izbyshev <izbyshev@ispras.ru>

Modules/cjkcodecs/_codecs_jp.c

@@ -40,7 +40,7 @@ ENCODER(cp932)
             if (c == 0xf8f0)
                 OUTBYTE1(0xa0);
             else
-                OUTBYTE1(c - 0xfef1 + 0xfd);
+                OUTBYTE1(c - 0xf8f1 + 0xfd);
             NEXT(1, 1);
             continue;
         }