[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/throw_exception_runme.ml

@@ -5,7 +5,7 @@ let x = new_Foo '()
 
 let check meth args expected =
   try
-    let _ = ((invoke x) meth (C_list [ args ])) in assert false
+    let _ = ((invoke x) meth (args)) in assert false
   with Failure msg -> assert (msg = expected)
 
 let _ =