Eliminate CSA false warning about null dereference in array_mark_proc

Eliminate CSA false warning about null dereference in array_mark_proc

The exact warning message is: Access to field 'mse_start' results in a
dereference of a null pointer (loaded from variable 'new_mark_stack_ptr').

* typd_mlc.c (GC_array_mark_proc): Add assertion that mark_stack_ptr is
non-NULL (thus orig_mark_stack_ptr is non-NULL too).