ICU-22913 Fix coverity warning in wrtxml.cpp #2878

Job	Run time
clang-valgrind-test	10m 19s
clang-valgrind-intltest (utility)	6m 50s
clang-valgrind-intltest (normalize)	6m 59s
clang-valgrind-intltest (collator)	5m 14s
clang-valgrind-intltest (regex)	5m 38s
clang-valgrind-intltest (format)	16m 8s
clang-valgrind-intltest (translit)	6m 8s
clang-valgrind-intltest (rbbi)	7m 33s
clang-valgrind-intltest (rbnf)	6m 22s
clang-valgrind-intltest (rbnfrt)	6m 38s
clang-valgrind-intltest (icuserv)	5m 10s
clang-valgrind-intltest (idna)	5m 33s
clang-valgrind-intltest (convert)	6m 51s
clang-valgrind-intltest (rbnfp)	5m 27s
clang-valgrind-intltest (csdet)	5m 11s
clang-valgrind-intltest (spoof)	5m 27s
clang-valgrind-intltest (bidi)	5m 30s
	1h 56m 58s

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