rts/m32: Fix assertion failure

This fixes an assertion failure in the m32 allocator due to the imprecisely specified preconditions of m32_allocator_push_filled_list. Specifically, the caller must ensure that the page type is set to filled prior to calling m32_allocator_push_filled_list.

While this issue did result in an assertion failure in the debug RTS, the issue is in fact benign.