ICU-22820 Fix memLeak during error in genrb #2480

Job	Run time
clang-valgrind-test	9m 53s
clang-valgrind-intltest (utility)	6m 37s
clang-valgrind-intltest (normalize)	6m 11s
clang-valgrind-intltest (collator)	5m 11s
clang-valgrind-intltest (regex)	5m 14s
clang-valgrind-intltest (format)	16m 33s
clang-valgrind-intltest (translit)	5m 39s
clang-valgrind-intltest (rbbi)	7m 19s
clang-valgrind-intltest (rbnf)	6m 20s
clang-valgrind-intltest (rbnfrt)	6m 3s
clang-valgrind-intltest (icuserv)	5m 8s
clang-valgrind-intltest (idna)	5m 21s
clang-valgrind-intltest (convert)	6m 36s
clang-valgrind-intltest (rbnfp)	4m 51s
clang-valgrind-intltest (csdet)	4m 50s
clang-valgrind-intltest (spoof)	5m 17s
clang-valgrind-intltest (bidi)	5m 14s
	1h 52m 17s

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