Check that buffer exists before trying to get its duration.

LayoutTests/webaudio/dom-exceptions.html

Index: LayoutTests/webaudio/dom-exceptions.html
diff --git a/LayoutTests/webaudio/dom-exceptions.html b/LayoutTests/webaudio/dom-exceptions.html
index 2725a705f1211b5438ce2ff0f06a28803db63329..f9b1e8458b496a44d886a198e4790838f1e2e9ea 100644
--- a/LayoutTests/webaudio/dom-exceptions.html
+++ b/LayoutTests/webaudio/dom-exceptions.html
@@ -121,7 +121,12 @@ function runTest() {
     // Start/stop for AudioBufferSourceNodes
     buffer = context.createBuffer(1,1, context.sampleRate);
     shouldNotThrow("source = context.createBufferSource()");
-    source.buffer = buffer;
+    shouldNotThrow("source.buffer = buffer");
+    shouldNotThrow("source.start()");
+    shouldNotThrow("source.stop()");
+
+    // It's valid to start a source that has no associated buffer.
+    shouldNotThrow("source = context.createBufferSource()");
     shouldNotThrow("source.start()");
     shouldNotThrow("source.stop()");