This patches exposed existing incorectness of swapcontext imlementation.
swapcontext does not set oucp->uc_stack. Unpoisoning works if ucp is
from makecontext, but may try to use garbage pointers if it's from
previos swapcontext or from getcontext. Existing limit reduces
probability of garbage pointers are used.
I restore behavour which we had for years, and will look to improve
swapcontext support.
This reverts commit d0751c9725aab1dad3d86481e13a4628356e7422.
