Fix http://bugs.busybox.net/view.php?id=826 in which "(echo; echo) | sed x"

crashes because the empty state of the hold space is NULL but the empty
state of the pattern space is "".

editors/sed.c

@@ -1054,7 +1054,7 @@ restart:
 					case 'x': /* Exchange hold and pattern space */
 					{
 						char *tmp = pattern_space;
-						pattern_space = bbg.hold_space;
+						pattern_space = bbg.hold_space ? : xzalloc(1);
 						no_newline=0;
 						bbg.hold_space = tmp;
 						break;