editor: Fix segfault when deleting the first character

clients/editor.c

@@ -380,11 +380,11 @@ text_input_keysym(void *data,
 		text_entry_commit_and_reset(entry);
 
 		start = utf8_prev_char(entry->text, entry->text + entry->cursor);
-		end = utf8_next_char(start);
-
 		if (start == NULL)
 			return;
 
+		end = utf8_next_char(start);
+
 		text_entry_delete_text(entry,
 				       start - entry->text,
 				       end - start);