Fix undefined behaviour in `scalar` `xnn_mul_s32`. #63

Job	Run time
build-and-test / cmake-android (build-android-arm64.sh)	11m 20s
build-and-test / cmake-linux-aarch64	10m 42s
build-and-test / cmake-android (build-android-armv7.sh)	10m 17s
build-and-test / cmake-linux-armhf	11m 42s
build-and-test / cmake-android (build-android-x86.sh)	11m 32s
build-and-test / cmake-linux-local	12m 47s
build-and-test / cmake-linux-riscv64	6m 29s
build-and-test / cmake-linux-riscv64-rvv	6m 45s
build-and-test / cmake-macos-arm64	10m 12s
build-and-test / cmake-windows-arm64	8m 28s
build-and-test / cmake-windows-x64	10m 28s
build-and-test / cmake-windows-x86	10m 51s
build-and-test / bazel-linux-aarch64	10m 20s
build-and-test / bazel-linux-x86_64	10m 26s
build-and-test / bazel-linux-x86_64-gcc-9	5m 34s
build-and-test / cmake-ios-arm64	0s
build-and-test / cmake-ios-x86_64	7m 38s
	2h 35m 31s

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