FIXED: Reduce risc of crashing when `hot-swapping' code in multi-threaded context.  Keri Harris.
FIXED: Reduce risc of crashing when `hot-swapping' code in multi-threaded
context.  Keri Harris.

jn7163/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel