virExec: avoid uninitialized memory usage

virExec: avoid uninitialized memory usage

valgrind warns:

==21079== Syscall param rt_sigaction(act->sa_mask) points to uninitialised byte(s)
==21079==    at 0x329840F63E: __libc_sigaction (sigaction.c:67)
==21079==    by 0x4E5A8E7: __virExec (util.c:661)

Regression introduced in commit ab07533e.  Technically, sa_mask
shouldn't affect operation if sa_flags selects sa_handler, and
sa_handler selects SIG_IGN, but better safe than sorry.

* src/util/util.c (__virExec): Supply missing sigemptyset.