Computer/CirclingComputer: Fix initialization of turnrate calculation

Fixes regression introduced in commit 55807f789bc8d73f9ce36ad21363f8a78a483020

==6525== Conditional jump or move depends on uninitialised value(s)
==6525==    at 0x433508: CirclingComputer::Turning(CirclingInfo&, MoreData const&, FlyingState const&, CirclingSettings const&) (CirclingComputer.cpp:143)
==6525==    by 0x43B0CD: GlideComputerAirData::Turning(MoreData const&, DerivedInfo&, ComputerSettings const&) (GlideComputerAirData.cpp:296)
==6525==    by 0x43A096: GlideComputerAirData::ProcessVertical(MoreData const&, DerivedInfo&, ComputerSettings const&) (GlideComputerAirData.cpp:96)
==6525==    by 0x437C2E: GlideComputer::ProcessGPS(bool) (GlideComputer.cpp:132)
==6525==    by 0x4B7366: CalculationThread::Tick() (CalculationThread.cpp:97)
==6525==    by 0x670953: WorkerThread::Run() (WorkerThread.cpp:66)
==6525==    by 0x670244: Thread::ThreadProc(void*) (Thread.cpp:145)
==6525==    by 0x4E3A0DA: start_thread (in /lib64/libpthread-2.18.so)
==6525==    by 0x6D2158C: clone (in /lib64/libc-2.18.so)
==6525==  Uninitialised value was created by a heap allocation
==6525==    at 0x4C27D49: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==6525==    by 0x4E7513: Startup() (Startup.cpp:321)
==6525==    by 0x5541E1: main (XCSoar.cpp:142)