Ensure GAL::SetGridSize( const VECTOR2D& aGridSize ) use non null sizes

aGridSize.x or aGridSize.y can be null if the config file has stored
0 as size.
This is illegal and crashed Kicad.
The min legal value is 1.0 in internal units

include/gal/graphics_abstraction_layer.h

@@ -881,6 +881,10 @@ public:
     {
         gridSize = aGridSize;
 
+        // Avoid stupid grid size values: a grid size  should be >= 1 in internal units
+        gridSize.x = std::max( 1.0, gridSize.x );
+        gridSize.y = std::max( 1.0, gridSize.y );
+
         gridOffset = VECTOR2D( (long) gridOrigin.x % (long) gridSize.x,
                                (long) gridOrigin.y % (long) gridSize.y );
     }