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/ODPCMAliasBlockFile.h

@@ -121,7 +121,7 @@ class ODPCMAliasBlockFile final : public PCMAliasBlockFile
                         size_t start, size_t len) const override;
 
    /// Read the summary into a buffer
-   bool ReadSummary(void *data) override;
+   bool ReadSummary(ArrayOf<char> &data) override;
 
    ///sets the file name the summary info will be saved in.  threadsafe.
    void SetFileName(wxFileNameWrapper &&name) override;