dwarf_getsrclines.c: Restore initialization of debug_str_offset

dwarf_getsrclines.c: Restore initialization of debug_str_offset

Prior to commit d4b0848b ("libdw: dwarf_getsrcfiles should not imply
dwarf_getsrclines") debug_str_offset was initialized with a default
value of 0.  This default initialization was removed as part of the
refactor introduced in d4b0848b.

Restore debug_str_offset's default initialization to 0.  For a
well-formed binary, it wasn't possible to use debug_str_offset
uninitialized.  The value is only relevant for line headers using
NVIDIA's CUBIN extension, in which case the real offset was always
stored in debug_str_offset before use.  However it's possible for a
malformed binary to cause uninitialized use and static analyzers
may complain about this.

Signed-off-by: Aaron Merey <amerey@redhat.com>