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.