Don't let the pattern space become null if the `x' command is used when the

hold space is null; some functions assume it's never null.

MFC after:	3 days

usr.bin/sed/process.c

@@ -237,6 +237,8 @@ process()
 					err(1, "%s", cp->t);
 				break;
 			case 'x':
+				if (hs == NULL)
+					cspace(&HS, "", 0, REPLACE);
 				tspace = PS;
 				PS = HS;
 				HS = tspace;