in_dxf: fix LTYPE.strings_area for r2007+ #3966

Job	Run time
ppc64-big-endian	6m 29s
linux-cmake	4m 22s
macOS	22m 51s
linux (CFLAGS="-O0 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindings)	7m 43s
mingw	22m 49s
linux (CC="clang")	9m 22s
mingw-cmake	7m 8s
linux (--enable-release)	19m 52s
mips32-big-endian	16m 48s
linux (--enable-gcov --disable-bindings)	14m 41s
vs2019	5m 13s
vs2019-ninja	4m 52s
linux (--disable-write)	8m 59s
linux (CFLAGS="-O2 -m32" --disable-bindings)	9m 48s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	16m 39s
	2h 57m 36s

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