smalloc: add a comment explaining why scalloc does not zero memory #855

Job	Run time
build (linux-gcc)	13m 13s
build (linux-clang)	13m 20s
build (macos)	15m 40s
build (linux-i686-gcc)	13m 20s
build (android)	39s
build (windows-cygwin-64)	16m 9s
build (windows-cygwin-32)	3m 48s
build (windows-msys2-64)	13m 12s
build (android-recovery, ubuntu-22.04, aarch64-linux-android32)	31s
	1h 29m 52s

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