Steps to reproduce (this PDF):
[page4.pdf](https://github.com/mozilla/pdf.js/files/5990272/page4.pdf)

````
npm install
node_modules/.bin/gulp dist-install
node examples/node/pdf2svg.js /tmp/page4.pdf
````

Error:
````
Error: Invalid SVG dimensions
    at unreachable (/tmp/pdf.js/build/dist/legacy/build/pdf.js:2086:9)
    at assert (/tmp/pdf.js/build/dist/legacy/build/pdf.js:2091:5)
    at DOMSVGFactory.create (/tmp/pdf.js/build/dist/legacy/build/pdf.js:640:24)
    at SVGGraphics._makeTilingPattern (/tmp/pdf.js/build/dist/legacy/build/pdf.js:24178:36)
    at SVGGraphics._makeColorN_Pattern (/tmp/pdf.js/build/dist/legacy/build/pdf.js:24127:23)
    at SVGGraphics.setFillColorN (/tmp/pdf.js/build/dist/legacy/build/pdf.js:24088:39)
    at SVGGraphics.executeOpTree (/tmp/pdf.js/build/dist/legacy/build/pdf.js:23634:22)
    at /tmp/pdf.js/build/dist/legacy/build/pdf.js:23508:18
    at process._tickCallback (internal/process/next_tick.js:68:7)
    at Function.Module.runMain (internal/modules/cjs/loader.js:834:11)
````
this.current.fillColor = this._makeColorN_Pattern(args);
    }
    shadingFill(args) {
      const width = this.viewport.width;
      const height = this.viewport.height;
      const inv = Util.inverseTransform(this.transformMatrix);
      const bl = Util.applyTransform([0, 0], inv);
      const br = Util.applyTransform([0, height], inv);
      const ul = Util.applyTransform([width, 0], inv);
      const ur = Util.applyTransform([width, height], inv);
      const x0 = Math.min(bl[0], br[0], ul[0], ur[0]);
      const y0 = Math.min(bl[1], br[1], ul[1], ur[1]);
      const x1 = Math.max(bl[0], br[0], ul[0], ur[0]);
      const y1 = Math.max(bl[1], br[1], ul[1], ur[1]);
      const rect = this.svgFactory.createElement("svg:rect");
      rect.setAttributeNS(null, "x", x0);
      rect.setAttributeNS(null, "y", y0);
      rect.setAttributeNS(null, "width", x1 - x0);
      rect.setAttributeNS(null, "height", y1 - y0);
      rect.setAttributeNS(null, "fill", this._makeShadingPattern(args));
      if (this.current.fillAlpha < 1) {
        rect.setAttributeNS(null, "fill-opacity", this.current.fillAlpha);
      }
      this._ensureTransformGroup().appendChild(rect);
    }
    /**
     * @private
     */
    _makeColorN_Pattern(args) {