sed: Fix backslash parsing for 'w' command arg

If there's any whitespace between w and the filename, parse_file_cmd
writes to the wrong offset when trying to fix up backslashes.

This can be seen in the asan build with busybox sed -e 'w 0\\'

Signed-off-by: Brian Foley <bpfoley@google.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

editors/sed.c

@@ -387,7 +387,7 @@ static int parse_file_cmd(/*sed_cmd_t *sed_cmd,*/ const char *filecmdstr, char *
 		bb_error_msg_and_die("empty filename");
 	*retval = xstrndup(filecmdstr+start, idx-start+hack+1);
 	if (hack)
-		(*retval)[idx] = '\\';
+		(*retval)[idx-start] = '\\';
 
 	return idx;
 }