libdw: Initialize tu_offset in __libdw_package_index

libdw: Initialize tu_offset in __libdw_package_index

dwarf_cu_dwp_section_info.c: In function ‘__libdw_package_index’:
dwarf_cu_dwp_section_info.c:306:25: error: ‘tu_offset’ may be used uninitialized [-Werror=maybe-uninitialized]
  306 |               tu_offset += tu_index->section_count * 4;
      |               ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
dwarf_cu_dwp_section_info.c:268:28: note: ‘tu_offset’ was declared here
  268 |       const unsigned char *tu_offset;
      |                            ^~~~~~~~~

Which is the same issue we thought to have fixed by checking for
tu_index != NULL but not all gcc versions seem able to see that.
So just explicitly initialize tu_offset to NULL. We keep the older
check, so the NULL pointer should never be used.

       * libdw/dwarf_cu_dwp_section_info.c (__libdw_package_index):
       Initialize tu_offset.

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