fix 2ndheader logging null-deref #18

Job	Run time
linux-cmake	4m 28s
linux (CFLAGS="-O0 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindings)	7m 43s
linux (CC="clang")	9m 27s
macOS	14m 36s
mingw	24m 30s
linux (--enable-release)	19m 43s
linux (--enable-gcov --disable-bindings)	14m 59s
mips32-big-endian	17m 2s
linux (--disable-write)	9m 17s
ppc64-big-endian	6m 37s
linux (CFLAGS="-O2 -m32" --disable-bindings)	9m 33s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	16m 25s
mingw-cmake	9m 14s
vs2019	4m 54s
vs2019-ninja	4m 46s
	2h 53m 14s

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