Prevent in Chrome pasting of text into upload panel

Change-Id: I20979ac6d426283ec099141ed01db01f91a61398
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

src/main/java/com/googlesource/gerrit/plugins/imagare/client/UploadByDropOrPastePanel.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/imagare/client/UploadByDropOrPastePanel.java b/src/main/java/com/googlesource/gerrit/plugins/imagare/client/UploadByDropOrPastePanel.java
index e034e69..7e559a6 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/imagare/client/UploadByDropOrPastePanel.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/imagare/client/UploadByDropOrPastePanel.java
@@ -63,12 +63,15 @@
           }
         }
 
-        if (blob !== null) {
+        if (blob) {
           var reader = new FileReader();
           reader.onload = function(e) {
             @com.googlesource.gerrit.plugins.imagare.client.ImageUploader::stageImage(Ljava/lang/String;)(e.target.result);
           };
           reader.readAsDataURL(blob);
+        } else {
+          e.preventDefault();
+          $wnd.Gerrit.showError('no image data');
         }
       } else if (e && e.clipboardData && e.clipboardData.getData) {
         // Webkit