GH-49884: [C++] Fix integer overflow in BufferBuilder reachable from JSON parsing #7879

Job	Run time
check-labels / Check labels	4s
check-enabled	2s
ODBC ${{ matrix.build-type }} ${{ matrix.architecture }} macOS ${{ matrix.macos-version }}	-1s
ARM64 Windows 11 MSVC	-1s
matrix.title	0s
ODBC Linux	-1s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	-1s
JNI macOS	-1s
ODBC Windows	0s
ODBC nightly	0s
report-extra-cpp	0s
	1s

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