libdw: Check terminating NUL byte in dwarf_getsrclines for dir/file table.

For DWARF version < 5 the .debug_line directory and file tables consist
of a terminating NUL byte after all strings. The code used to just skip
this without checking it actually existed. This could case a spurious
read past the end of data.

Fix the same issue in readelf.

https://sourceware.org/bugzilla/show_bug.cgi?id=24102



Signed-off-by: Mark Wielaard <mark@klomp.org>