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/stringdefs.h b/Objects/stringlib/stringdefs.h
index 9619332..41449fc 100644
--- a/Objects/stringlib/stringdefs.h
+++ b/Objects/stringlib/stringdefs.h
@@ -9,6 +9,7 @@
 #define FASTSEARCH fastsearch
 #define STRINGLIB(F) stringlib_##F
 #define STRINGLIB_OBJECT         PyBytesObject
+#define STRINGLIB_SIZEOF_CHAR    1
 #define STRINGLIB_CHAR           char
 #define STRINGLIB_TYPE_NAME      "string"
 #define STRINGLIB_PARSE_CODE     "S"