Hi,

This affects every version with the --debug flag to my knowledge. Tested 
on version 4.8. Reasonably simple reproducer is attached. Run with `sed 
-f repro.sed --debug`.

I believe the root cause is that sed will compile scripts *before* 
setting the debug flag, which leads to cmd->x.label_name being garbage 
since next_cmd_entry doesn't zero out the auxiliary data structure. When 
sed then tries to print the label through debug_print_program at the end 
of main, a segfault is possible due to the uninitialized read.