HpAlloc was not being set when returning to the scheduler via MAYBE_GC(),
which at the least was just wrong (the scheduler might allocate a large
block more than once), and at worst could lead to crashes if HpAlloc contains
garbage.
Fixes at least one threaded2 test on Windows.
if (bdescr_link(CurrentNursery) == NULL || CInt[alloc_blocks] >= CInt[alloc_blocks_lim]) { \
R9 = liveness; \
R10 = reentry; \
if (bdescr_link(CurrentNursery) == NULL || CInt[alloc_blocks] >= CInt[alloc_blocks_lim]) { \
R9 = liveness; \
R10 = reentry; \