Ensure no garbage in struct sigaction.
Valgrind was complaining:

==21435== Syscall param rt_sigaction(act->sa_mask) points to uninitialised byte(s)
==21435==    at 0x95ED000: __libc_sigaction (in /usr/lib64/libc-2.27.so)
==21435==    by 0x594E99: main
==21435==  Address 0x1ffefff8e8 is on thread 1's stack
==21435==  in frame #0, created by __libc_sigaction (???:)