Fix signed ints #19

Job	Run time
linux-cmake	5m 22s
mingw	31m 19s
macOS	12m 26s
linux (CFLAGS="-O1 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindin...	15m 17s
mingw-cmake	7m 19s
linux (CC="clang")	14m 29s
ppc64-big-endian	14m 21s
mips32-big-endian	31m 35s
linux (--enable-release)	31m 17s
vs2019	5m 2s
linux (--enable-gcov --disable-bindings --disable-docs CFLAGS="-O0 -g")	7m 55s
vs2019-ninja	4m 23s
linux (--disable-write --disable-docs)	13m 28s
linux (CFLAGS="-O2 -m32" --disable-bindings --disable-docs)	12m 54s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	31m 17s
	3h 58m 24s

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