do_checks: do not set HpAlloc if the stack check fails
This fixes a very rare heap corruption bug, whereby - a context switch is requested, which sets HpLim to zero (contextSwitchCapability(), called by the timer signal or another Capability). - simultaneously a stack check fails, in a code fragment that has both a stack and a heap check. The RTS then assumes that a heap-check failure has occurred and subtracts HpAlloc from Hp, although in fact it was a stack-check failure and retreating Hp will overwrite valid heap objects. The bug is that HpAlloc should only be set when Hp has been incremented by the heap check. See comments in rts/HeapStackCheck.cmm for more details. This bug is probably incredibly rare in practice, but I happened to be working on a test that triggers it reliably: concurrent/should_run/throwto001, compiled with -O -threaded, args 30 300 +RTS -N2, run repeatedly in a loop.
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