rts: allow NULL to be used as an invalid StgStablePtr
Previously you could have a NULL stable ptr that would dereference to a valid closure that you might not expect, like in getTopHandlerThread where NULL meant uninitialised.
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add some tests perhaps -
write the note -
check if there are more cases like getTopHandlerThread where we should check the returned value -
longer commit message, reference #20998
Edited by adam