Ensure that we don't reset the current page view if the user inputs an invalid page number (PR 7289 follow-up)

After PR 7289, we'll now reset the current page view in cases where I don't think we should. To avoid this, this patch ensures that we'll not modify the position when the page number is out-of-bounds.

**STR:**
1. Open http://mozilla.github.io/pdf.js/web/viewer.html#page=1&zoom=auto,-98,696
2. Enter an invalid number, e.g. `1000`, in the `pageNumber` input.

**ER:**
The current position in the document shouldn't change, since the page number wasn't valid.

**AR:**
The document resets to the top of the page `1`.

web/pdf_viewer.js

@@ -155,22 +155,34 @@ var PDFViewer = (function pdfViewer() {
       return this._pages[index];
     },
 
+    /**
+     * @returns {number}
+     */
     get currentPageNumber() {
       return this._currentPageNumber;
     },
 
+    /**
+     * @param {number} val - The page number.
+     */
     set currentPageNumber(val) {
       if (!this.pdfDocument) {
         this._currentPageNumber = val;
         return;
       }
-      this._setCurrentPageNumber(val);
       // The intent can be to just reset a scroll position and/or scale.
-      this._resetCurrentPageView();
+      this._setCurrentPageNumber(val, /* resetCurrentPageView = */ true);
     },
 
-    _setCurrentPageNumber: function pdfViewer_setCurrentPageNumber(val) {
+    /**
+     * @private
+     */
+    _setCurrentPageNumber:
+        function pdfViewer_setCurrentPageNumber(val, resetCurrentPageView) {
       if (this._currentPageNumber === val) {
+        if (resetCurrentPageView) {
+          this._resetCurrentPageView();
+        }
         return;
       }
       var arg;
@@ -193,6 +205,10 @@ var PDFViewer = (function pdfViewer() {
       this._currentPageNumber = val;
       this.eventBus.dispatch('pagechanging', arg);
       this.eventBus.dispatch('pagechange', arg);
+
+      if (resetCurrentPageView) {
+        this._resetCurrentPageView();
+      }
     },
 
     /**
@@ -500,6 +516,7 @@ var PDFViewer = (function pdfViewer() {
 
     /**
      * Refreshes page view: scrolls to the current page and updates the scale.
+     * @private
      */
     _resetCurrentPageView: function () {
       if (this.isInPresentationMode) {
@@ -524,8 +541,7 @@ var PDFViewer = (function pdfViewer() {
       }
 
       if (this.isInPresentationMode || !dest) {
-        this._setCurrentPageNumber(pageNumber);
+        this._setCurrentPageNumber(pageNumber, /* resetCurrentPageView */ true);
-        this._resetCurrentPageView();
         return;
       }