Use at least 1x1 pixel canvas for groups.

src/canvas.js

@@ -1422,9 +1422,10 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
                     group.bbox,
                     currentCtx.mozCurrentTransform);
       // Use ceil in case we're between sizes so we don't create canvas that is
-      // too small.
+      // too small and make the canvas at least 1x1 pixels.
-      var drawnWidth = Math.ceil(bounds[2] - bounds[0]);
+      var drawnWidth = Math.max(Math.ceil(bounds[2] - bounds[0]), 1);
-      var drawnHeight = Math.ceil(bounds[3] - bounds[1]);
+      var drawnHeight = Math.max(Math.ceil(bounds[3] - bounds[1]), 1);
+
       var scratchCanvas = createScratchCanvas(drawnWidth, drawnHeight);
       var groupCtx = scratchCanvas.getContext('2d');
       addContextCurrentTransform(groupCtx);