dxf: fix dxf_fixup_string logic #4598

Job	Run time
ppc64-big-endian	7m 32s
linux (CFLAGS="-O1 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindin...	3m 56s
linux (CC="clang")	4m 0s
linux (--enable-release)	36m 5s
linux (--enable-gcov --disable-bindings --disable-docs CFLAGS="-O0 -g")	5m 21s
linux (--disable-write --disable-docs)	5m 31s
linux-cmake	2m 2s
macOS	5m 24s
linux (CFLAGS="-O2 -m32" --disable-bindings --disable-docs)	4m 19s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	9m 7s
mingw	29m 58s
mingw-cmake	2m 53s
mips32-big-endian	8m 32s
vs2019	6m 40s
vs2019-ninja	4m 9s
	2h 15m 29s

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