Invalid use of assert() around expressions with required side-effects (i.e., getcontext())

If the assertions are compiled out, the calls to getcontext will be compiled out also...
This probably doesn't affect Ubuntu builds, since the package is never built with -DNDEBUG by default (I think).

But strictly speaking it's not correct and it will probably only be a matter of time before it confuses someone (like me :P )

I checked through other uses of assert in the code, and it looks like the only affected code is where assert is used around getcontext calls in utils/wvtask.cc.

The attached patch can work around the issue, though it is not the most elegant possible solution. The fix should also be pushed upstream if it is considered relevant.