Fix infinite loop when painting grid with bad parameters

Credit to @m-kuhn

Cherry-picked from 4876c65

Author: nyalldawson

src/core/composer/qgscomposermapgrid.cpp

@@ -1644,6 +1644,9 @@ int QgsComposerMapGrid::xGridLinesCRSTransform( const QgsRectangle& bbox, const
     step = ( maxX + 360.0 - minX ) / 20;
   }
 
+  if ( step == 0 )
+    return 1;
+
   int gridLineCount = 0;
   while ( currentLevel >= bbox.yMinimum() && gridLineCount < MAX_GRID_LINES )
   {
@@ -1707,6 +1710,9 @@ int QgsComposerMapGrid::yGridLinesCRSTransform( const QgsRectangle& bbox, const
   double maxY = bbox.yMaximum();
   double step = ( maxY - minY ) / 20;
 
+  if ( step == 0 )
+    return 1;
+
   bool crosses180 = false;
   bool crossed180 = false;
   if ( mCRS.geographicFlag() && ( bbox.xMinimum() > bbox.xMaximum() ) )