[OCaml] Fix segfaults when too few arguments are passed to a function

Prevent segfaults when too few arguments are passed to a function.

Length checks are not needed for the wrappers of overloaded
functions -- the generated dispatch function already checks.

Add default_args_runme.ml.

Fix minor errors in some runtime tests.  Extra args were being passed
in some cases.

Examples/test-suite/ocaml/exception_order_runme.ml

@@ -5,7 +5,7 @@ let a = new_A '()
 
 let check meth args expected =
   try
-    ignore ((invoke a) meth (C_list [ args ])); assert false
+    ignore ((invoke a) meth (args)); assert false
   with Failure msg -> assert (msg = expected)
 
 let _ =