Improve initialization of struct for stack trace

[theoreticalbts]

@vogel76 suggests the following:

I think initialization shall be done as follows:

   struct sigaction sigact = {0};
   sigset_t block_mask = {0};
   sigemptyset (&block_mask);
   
   sigact.sa_sigaction = segfault_handler;
   sigact.sa_flags = SA_RESTART | SA_SIGINFO;
   sigact.sa_mask = block_mask;

Valgrind complains about sa_mask field, and I think it is dangerous to leave trash there. Also I am not sure if inside this handler SIGINT shall be blocked to avoid spawning standard break handler during execution of SIGSEGV