Check that buffer exists before trying to get its duration.

LayoutTests/webaudio/dom-exceptions-expected.txt

Index: LayoutTests/webaudio/dom-exceptions-expected.txt
diff --git a/LayoutTests/webaudio/dom-exceptions-expected.txt b/LayoutTests/webaudio/dom-exceptions-expected.txt
index 6d2b172198c4dd666a119f75a87d9394e0fe4246..604f309aa4a4e7afed15e6cad7b1840b5bb79950 100644
--- a/LayoutTests/webaudio/dom-exceptions-expected.txt
+++ b/LayoutTests/webaudio/dom-exceptions-expected.txt
@@ -46,6 +46,10 @@ PASS new webkitOfflineAudioContext(1, 100, 1) threw exception SyntaxError: Faile
 PASS new webkitOfflineAudioContext(1, 100, 1e6) threw exception SyntaxError: Failed to construct 'OfflineAudioContext': sample rate (1.00000e+6) must be in the range 44100-96000 Hz..
 PASS node.oversample = '9x' threw exception InvalidStateError: Failed to set the 'oversample' property on 'WaveShaperNode': invalid oversample '9x': must be 'none', '2x', or '4x'..
 PASS source = context.createBufferSource() did not throw exception.
+PASS source.buffer = buffer did not throw exception.
+PASS source.start() did not throw exception.
+PASS source.stop() did not throw exception.
+PASS source = context.createBufferSource() did not throw exception.
 PASS source.start() did not throw exception.
 PASS source.stop() did not throw exception.
 PASS source = context.createOscillator() did not throw exception.