Fix a potential divided by O error

lib/screenshotHandler.js

@@ -54,19 +54,24 @@ var screenshotHandler = function() {
         var deferred = Q.defer();
 
         var currentWidth = image.bitmap.width;
-        var ratio = newWidth / currentWidth;
 
-        image.scale(ratio, function(err, image){
-            if (err) {
-                debug('Could not resize image');
-                debug(err);
-
-                deferred.reject(err);
-            } else {
-                debug('Image correctly resized');
-                deferred.resolve(image);
-            }
-        });
+        if (currentWidth > 0) {
+            var ratio = newWidth / currentWidth;
+
+            image.scale(ratio, function(err, image){
+                if (err) {
+                    debug('Could not resize image');
+                    debug(err);
+
+                    deferred.reject(err);
+                } else {
+                    debug('Image correctly resized');
+                    deferred.resolve(image);
+                }
+            });
+        } else {
+            deferred.reject('Could not resize an empty image');
+        }
 
         return deferred.promise;        
     };