initialize curFPS to a reasonable value

Fixes audioGetNumSamplesOut sometimes breaking audio completely on start
because of using the value uninitialized.

Full credit to Arisotura for somehow figuring this one out

Author: nadiaholmquist

src/frontend/qt_sdl/EmuInstance.cpp

@@ -99,6 +99,7 @@ EmuInstance::EmuInstance(int inst) : deleting(false),
         targetFPS = 60.0;
     }
     else targetFPS = val;
+    curFPS = targetFPS;
 
     val = globalCfg.GetDouble("FastForwardFPS");
     if (val == 0.0)