[apps/continuous_function_cache] Clear on bad step

Cache did not check that the step was the same before panning, causing
old values to remains cached.
e.g. :
  - Type f(x) = x, and plot it.
  - Go to Axes, and set Xmax = 0.
  - Confirm

Change-Id: Ie8ed10c336cf517a65f19b05bd14866daf8128c2

apps/shared/continuous_function_cache.cpp

@@ -31,6 +31,10 @@ void ContinuousFunctionCache::PrepareForCaching(void * fun, ContinuousFunctionCa
     function->setCache(cache);
   }
 
+  if (tStep != 0. && tStep != cache->step()) {
+    cache->clear();
+  }
+
   if (function->plotType() == ContinuousFunction::PlotType::Cartesian && tStep != 0) {
     function->cache()->pan(function, tMin);
   }