debugedit: Make sure .debug_line old/new idx start equal.

Found by running the debugedit tests under valgrind.
If the old and new .debug_line offset isn't changed then we might
write out an uninitialized new_idx.

tools/debugedit.c

@@ -1177,6 +1177,7 @@ get_line_table (DSO *dso, size_t off, struct line_table **table)
   *table = NULL;
 
   t->old_idx = off;
+  t->new_idx = off;
   t->size_diff = 0;
   t->replace_dirs = false;
   t->replace_files = false;