Ensure that chart dimensions are always >= 0

Elements were resizing incorrectly if they were regenerated while the chart was in a div that was display:none. Added a check to avoid this issue. Resolves #4397

src/core/core.controller.js

@@ -169,8 +169,10 @@ module.exports = function(Chart) {
 
 			// the canvas render width and height will be casted to integers so make sure that
 			// the canvas display style uses the same integer values to avoid blurring effect.
-			var newWidth = Math.floor(helpers.getMaximumWidth(canvas));
-			var newHeight = Math.floor(aspectRatio? newWidth / aspectRatio : helpers.getMaximumHeight(canvas));
+
+            // Set to 0 instead of canvas.size because the size defaults to 300x150 if the element is collased
+			var newWidth = Math.max(0, Math.floor(helpers.getMaximumWidth(canvas)));
+			var newHeight = Math.max(0, Math.floor(aspectRatio ? newWidth / aspectRatio : helpers.getMaximumHeight(canvas)));
 
 			if (me.width === newWidth && me.height === newHeight) {
 				return;