FIX (partially) #2703: bug in stack overflow handling when inside block

FIX (partially) #2703: bug in stack overflow handling when inside block
As a result of a previous ticket (#767) we disabled the generation of
StackOverflow exceptions when inside a Control.Exception.block, on the
grounds that StackOverflow is like an asynchronous exception.  Instead
we just increase the stack size.  However, the stack size calculation
was wrong, and ended up not increasing the size of the stack, with the
result that the runtime just kept re-allocating the stack and filling
up memory.