DebugTools: avoid accidental OOB access in FrameProfiler internals.

The memory wasn't accessed in that case so everything is fine, but this
started tripping up the new assertions in ArrayView element access.
Fixed by moving (and thus duplicating) the access to where it is
actually needed.

Also removed the outdated comment -- there's no _currentData anymore
anywhere, not sure what was that meant to be.

src/Magnum/DebugTools/FrameProfiler.cpp

@@ -190,21 +190,17 @@ void FrameProfiler::endFrame() {
         Measurement& measurement = _measurements[i];
         const UnsignedInt measurementDelay = Math::max(1u, measurement._delay);
 
-        /* Where to save currently queried data. For _delay of 0 or 1,
-           delayedCurrentData(Math::max(1u, measurement._delay)) is equal to
-           _currentData. */
-        UnsignedLong& currentMeasurementData = _data[delayedCurrentData(measurementDelay)*_measurements.size() + i];
-
         /* If we're wrapping around, subtract the oldest data from the moving
            average so we can reuse the memory for currently queried data */
         if(_measuredFrameCount > _maxFrameCount + measurementDelay - 1) {
+            UnsignedLong& currentMeasurementData = _data[delayedCurrentData(measurementDelay)*_measurements.size() + i];
             CORRADE_INTERNAL_ASSERT(measurement._movingSum >= currentMeasurementData);
             measurement._movingSum -= currentMeasurementData;
         }
 
         /* Simply save the data if not delayed */
         if(!measurement._delay)
-            currentMeasurementData = measurement._query.immediate(measurement._state);
+            _data[delayedCurrentData(measurementDelay)*_measurements.size() + i] = measurement._query.immediate(measurement._state);
 
         /* For delayed measurements call the end function for current frame and
            then save the data for the delayed frame */
@@ -215,7 +211,7 @@ void FrameProfiler::endFrame() {
                reused for a a new value next frame */
             const UnsignedInt previous = (measurement._current + 1) % measurement._delay;
             if(_measuredFrameCount >= measurement._delay) {
-                currentMeasurementData =
+                _data[delayedCurrentData(measurementDelay)*_measurements.size() + i] =
                     measurement._query.delayed(measurement._state, previous, measurement._current);
             }
             measurement._current = previous;
@@ -228,9 +224,8 @@ void FrameProfiler::endFrame() {
         Measurement& measurement = _measurements[i];
         const UnsignedInt measurementDelay = Math::max(1u, measurement._delay);
 
-        /* If we have enough frames, add the new measurement to the moving sum.
-           For _delay of 0 or 1, delayedCurrentData(Math::max(1u, measurement._delay))
-           is equal to _currentData. */
+        /* If we have enough frames, add the new measurement to the moving
+           sum */
         if(_measuredFrameCount >= measurementDelay) {
             const UnsignedLong data = _data[delayedCurrentData(measurementDelay)*_measurements.size() + i];
             CORRADE_INTERNAL_ASSERT(_measurements[i]._movingSum + data >= _measurements[i]._movingSum);