Fix two copy/paste bugs: forbid zero-length buffers to prevent a fatal error

when trying to paste them, found by me, and miscalculation of the start/end
causing random fatal errors when copying in copy-mode, reported by sthen.

ok sthen "put it in" deraadt

paste.c

@@ -101,6 +101,9 @@ paste_add(struct paste_stack *ps, char *data, u_int limit)
 {
 	struct paste_buffer	*pb;
 
+	if (*data == '\0')
+		return;
+
 	while (ARRAY_LENGTH(ps) >= limit)
 		ARRAY_TRUNC(ps, 1);