Fix [63325009a8]: \u0400 conversion to and from cesu-8 fails. #4531

Job	Run time
gcc	3m 42s
gcc (CFLAGS=-DTCL_NO_DEPRECATED=1)	3m 43s
gcc (--disable-shared)	3m 45s
gcc (--disable-zipfs)	3m 41s
gcc (--enable-symbols)	3m 42s
gcc (--enable-symbols=mem)	5m 3s
gcc (--enable-symbols=all)	6m 26s
gcc (CFLAGS=-ftrapv)	3m 48s
gcc (CFLAGS=-m32 CPPFLAGS=-m32 LDFLAGS=-m32 --disable-64bit)	4m 5s
	37m 55s

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