MINOR: [C++] Avoid multimap::find unspecified behavior #3976

Job	Run time
check-labels / Check labels	29s
ODBC	0s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	0s
matrix.title	0s
ARM64 Windows 11 MSVC	-1s
JNI macOS	-1s
report-extra-cpp	-1s
	26s

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