Fix fastsearch for UCS2 and UCS4

 * If needle is 0, try (p[0] >> 16) & 0xff for UCS4
 * Disable fastsearch_memchr_1char() if needle is zero for UCS2 and UCS4
diff --git a/Objects/stringlib/undef.h b/Objects/stringlib/undef.h
index 40b4391..fd87e56 100644
--- a/Objects/stringlib/undef.h
+++ b/Objects/stringlib/undef.h
@@ -1,5 +1,6 @@
 #undef  FASTSEARCH
 #undef  STRINGLIB
+#undef  STRINGLIB_SIZEOF_CHAR
 #undef  STRINGLIB_CHAR
 #undef  STRINGLIB_STR
 #undef  STRINGLIB_LEN