[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/global_ns_arg_runme.ml

@@ -2,4 +2,4 @@ open Swig
 open Global_ns_arg
 
 let _ = assert ((_foo '(1) as int) = 1)
-let _ = assert ((_bar_fn '(1) as int) = 1)
+let _ = assert ((_bar_fn '() as int) = 1)