Fix [63325009a8]: \u0400 conversion to and from cesu-8 fails #4518

Job	Run time
msvc	10m 33s
gcc	12m 21s
msvc (CHECKS=nodep)	10m 27s
gcc (CFLAGS=-DTCL_NO_DEPRECATED=1)	12m 37s
msvc (OPTS=static)	10m 37s
gcc (--disable-shared)	12m 53s
msvc (OPTS=noembed)	10m 29s
gcc (--disable-zipfs)	12m 29s
msvc (OPTS=symbols)	11m 16s
gcc (--enable-symbols)	12m 48s
msvc (OPTS=symbols STATS=compdbg,memdbg)	16m 34s
gcc (--enable-symbols=mem)	14m 19s
gcc (--enable-symbols=all)	15m 50s
	2h 43m 13s

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