rts: avoid checking bdescr of value outside of Haskell heap

In nonmovingTidyWeaks we want to check if the key of a weak pointer
lives in the non-moving heap. We do this by checking the flags of the
block the key lives in. But we need to be careful with values that live
outside the Haskell heap, since they will lack a block descriptor and
looking for one may lead to a segfault. In this case we should just
accept that it isn't on the non-moving heap.

Resolves #24492