Core: QUnit.start fails if called with a non-numeric argument

Fixes #846
Closes #877

src/core.js

@@ -111,6 +111,17 @@ extend( QUnit, {
 			// If a test is running, adjust its semaphore
 			config.current.semaphore -= count || 1;
 
+			// If semaphore is non-numeric, throw error
+			if ( isNaN( config.current.semaphore ) ) {
+				config.current.semaphore = 0;
+
+				QUnit.pushFailure(
+					"Called start() with a non-numeric decrement.",
+					sourceFromStacktrace( 2 )
+				);
+				return;
+			}
+
 			// Don't start until equal number of stop-calls
 			if ( config.current.semaphore > 0 ) {
 				return;

test/main/async.js

@@ -94,6 +94,19 @@ QUnit.test( "fails if start is called more than stop", function( assert ) {
 		"already\\)" ) );
 });
 
+QUnit.test( "fails if start is called with a non-numeric argument", function( assert ) {
+	QUnit.stop();
+
+	// Duck-punch to force an Error to be thrown instead of a `pushFailure` call
+	assert.test.pushFailure = function( msg ) {
+		throw new Error( msg );
+	};
+
+	assert.throws(function() {
+		QUnit.start( "ok" );
+	}, /Called start\(\) with a non\-numeric decrement\./ );
+});
+
 QUnit.module( "asyncTest" );
 
 QUnit.asyncTest( "asyncTest", function( assert ) {