Return ptrace event message when there's no error instead of

only when there's an error.

R=rsc
APPROVED=rsc
DELTA=3  (0 added, 2 deleted, 1 changed)
OCL=31650
CL=31650

src/pkg/syscall/syscall_linux.go

@@ -521,9 +521,7 @@ func PtraceSetOptions(pid int, options int) (errno int) {
 func PtraceGetEventMsg(pid int) (msg uint, errno int) {
 	var data _C_long;
 	errno = ptrace(_PTRACE_GETEVENTMSG, pid, 0, uintptr(unsafe.Pointer(&data)));
-	if errno != 0 {
 	msg = uint(data);
-	}
 	return;
 }