Callbacks: Disabling a callback should prevent firing

Thanks to @TheDistantSea for the report!

Fixes gh-1790
Closes gh-1643
(cherry picked from commit bc1cb122db30be034b3da84ee2546c86e2a5268f)

src/callbacks.js

@@ -151,8 +151,10 @@ jQuery.Callbacks = function( options ) {
 			},
 			// Remove all callbacks from the list
 			empty: function() {
-				list = [];
-				firingLength = 0;
+				if ( list ) {
+					list = [];
+					firingLength = 0;
+				}
 				return this;
 			},
 			// Have the list do nothing anymore

test/unit/callbacks.js

@@ -340,3 +340,18 @@ test( "jQuery.Callbacks() - adding a string doesn't cause a stack overflow", fun
 
 	ok( true, "no stack overflow" );
 });
+
+test( "jQuery.Callbacks() - disabled callback doesn't fire (gh-1790)", function() {
+
+	expect( 1 );
+
+	var cb = jQuery.Callbacks(),
+		fired = false,
+		shot = function() { fired = true; };
+
+	cb.disable();
+	cb.empty();
+	cb.add( shot );
+	cb.fire();
+	ok( !fired, "Disabled callback function didn't fire" );
+});