Fix small kernel memory disclosures. [EN-18:12.mem]

Reported by:	Thomas Barabosch, Fraunhofer FKIE
Approved by:	so
Security:	FreeBSD-EN-18:12.mem
Security:	CVE-2018-17155

Author: tetlowgm

sys/kern/kern_context.c

@@ -68,6 +68,7 @@ sys_getcontext(struct thread *td, struct getcontext_args *uap)
 	if (uap->ucp == NULL)
 		ret = EINVAL;
 	else {
+		bzero(&uc, sizeof(ucontext_t));
 		get_mcontext(td, &uc.uc_mcontext, GET_MC_CLEAR_RET);
 		PROC_LOCK(td->td_proc);
 		uc.uc_sigmask = td->td_sigmask;
@@ -108,6 +109,7 @@ sys_swapcontext(struct thread *td, struct swapcontext_args *uap)
 	if (uap->oucp == NULL || uap->ucp == NULL)
 		ret = EINVAL;
 	else {
+		bzero(&uc, sizeof(ucontext_t));
 		get_mcontext(td, &uc.uc_mcontext, GET_MC_CLEAR_RET);
 		bzero(uc.__spare__, sizeof(uc.__spare__));
 		PROC_LOCK(td->td_proc);