find: When performing -quit, finish pending -exec ... + command lines.

This avoids unexpected partial processing when a find command uses both
-quit and -exec ... +.

GNU find does the same.

MFC after:	1 week

usr.bin/find/function.c

@@ -1774,6 +1774,7 @@ f_false(PLAN *plan __unused, FTSENT *entry __unused)
 int
 f_quit(PLAN *plan __unused, FTSENT *entry __unused)
 {
+	finish_execplus();
 	exit(0);
 }