ICU-22602 Fix stack overflow inside flattenVariables #1324

Job	Run time
clang-valgrind-test	10m 18s
clang-valgrind-intltest (utility)	6m 52s
clang-valgrind-intltest (normalize)	6m 38s
clang-valgrind-intltest (collator)	5m 2s
clang-valgrind-intltest (regex)	5m 17s
clang-valgrind-intltest (format)	16m 41s
clang-valgrind-intltest (translit)	5m 54s
clang-valgrind-intltest (rbbi)	7m 14s
clang-valgrind-intltest (rbnf)	6m 8s
clang-valgrind-intltest (rbnfrt)	6m 19s
clang-valgrind-intltest (icuserv)	5m 8s
clang-valgrind-intltest (idna)	5m 13s
clang-valgrind-intltest (convert)	6m 24s
clang-valgrind-intltest (rbnfp)	5m 11s
clang-valgrind-intltest (csdet)	5m 17s
clang-valgrind-intltest (spoof)	5m 49s
clang-valgrind-intltest (bidi)	5m 41s
	1h 55m 6s

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