Bug 596429 - Firefox 4.0b5 Crash Report nsFastLoadFileWriter::Open() r=benedict a=2.0

xpcom/io/nsFastLoadFile.cpp

@@ -1852,6 +1852,9 @@ nsFastLoadFileWriter::Open()
 {
     nsresult rv;
     
+    if (!mSeekableOutput)
+        return NS_ERROR_FAILURE;
+
     rv = mSeekableOutput->Seek(nsISeekableStream::NS_SEEK_SET,
                                sizeof(nsFastLoadHeader));
     if (NS_FAILED(rv))