Check that buffer exists before trying to get its duration.

Source/modules/webaudio/AudioBufferSourceNode.cpp

Index: Source/modules/webaudio/AudioBufferSourceNode.cpp
diff --git a/Source/modules/webaudio/AudioBufferSourceNode.cpp b/Source/modules/webaudio/AudioBufferSourceNode.cpp
index 331d300ab44fef25769b71c359efcfcf1b4f5c05..875621293829a4497b5a690a4f68d3f1a0765e2b 100644
--- a/Source/modules/webaudio/AudioBufferSourceNode.cpp
+++ b/Source/modules/webaudio/AudioBufferSourceNode.cpp
@@ -383,12 +383,12 @@ unsigned AudioBufferSourceNode::numberOfChannels()
 
 void AudioBufferSourceNode::start(double when)
 {
-    startPlaying(false, when, 0, buffer()->duration());
+    startPlaying(false, when, 0, buffer() ? buffer()->duration() : 0);
 }
 
 void AudioBufferSourceNode::start(double when, double grainOffset)
 {
-    startPlaying(true, when, grainOffset, buffer()->duration());
+    startPlaying(true, when, grainOffset, buffer() ? buffer()->duration() : 0);
 }
 
 void AudioBufferSourceNode::start(double when, double grainOffset, double grainDuration)