Zero and return false for all failures to read block file summary...

... Though in the only place where these summaries are used, which is
Sequence::GetWaveDisplay, we ignore the correctly reported error code anyway.

Also RAII in management of relevant memory buffers and mutexes.

src/blockfile/SilentBlockFile.cpp

@@ -24,9 +24,10 @@ SilentBlockFile::~SilentBlockFile()
 {
 }
 
-bool SilentBlockFile::ReadSummary(void *data)
+bool SilentBlockFile::ReadSummary(ArrayOf<char> &data)
 {
-   memset(data, 0, mSummaryInfo.totalSummaryBytes);
+   data.reinit( mSummaryInfo.totalSummaryBytes );
+   memset(data.get(), 0, mSummaryInfo.totalSummaryBytes);
    return true;
 }