decode_r2007: fix heap overflow by one #3842

Job	Run time
macOS	15m 3s
linux (CFLAGS="-O0 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindings)	7m 48s
linux-cmake	4m 19s
linux (CC="clang")	9m 40s
mingw	24m 7s
linux (--enable-release)	20m 16s
mingw-cmake	7m 17s
linux (--enable-gcov --disable-bindings)	14m 55s
mips32-big-endian	16m 51s
ppc64-big-endian	6m 48s
linux (--disable-write)	8m 54s
vs2019	5m 22s
linux (CFLAGS="-O2 -m32" --disable-bindings)	9m 47s
vs2019-ninja	4m 29s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	16m 36s
	2h 52m 12s

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