Turn some unreachable return statements into reachable (to suppress

Turn some unreachable return statements into reachable (to suppress
compiler warning)

* dbg_mlc.c (GC_generate_random_heap_address): Change function
algorithm structure to have reachable return statement at function end.
* mark.c (GC_mark_some): Likewise.
* os_dep.c (GC_mprotect_thread): Place a dummy return statement (which
uses function "arg"); remove unreachable return statement.
* win32_threads.c (GC_DllMain): Place GC_ASSERT before
GC_get_stack_base.