compositor: initialise segv_action.sa_mask

Valgrind complained sa_mask member is not initialised, fix that.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/src/compositor.c b/src/compositor.c
index 5b5bcc7..b4c766c 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -2133,6 +2133,7 @@
 
 	segv_action.sa_flags = SA_SIGINFO | SA_RESETHAND;
 	segv_action.sa_sigaction = on_segv_signal;
+	sigemptyset(&segv_action.sa_mask);
 	sigaction(SIGSEGV, &segv_action, NULL);
 
 	if (!backend) {