Update documentation and add checks surrounding block files

(since we now have silent block files with no disk file)

src/BlockFile.cpp

@@ -114,7 +114,9 @@ BlockFile::~BlockFile()
 
 /// Returns the file name of the disk file associated with this
 /// BlockFile.  Not all BlockFiles store their sample data here,
-/// but all BlockFiles have at least their summary data here.
+/// but most BlockFiles have at least their summary data here.
+/// (some, i.e. SilentBlockFiles, do not correspond to a file on
+///  disk and have empty file names)
 wxFileName BlockFile::GetFileName()
 {
    return mFileName;

src/BlockFile.h

@@ -77,6 +77,8 @@ class BlockFile {
    virtual void SaveXML(XMLWriter &xmlFile) = 0;
 
    /// Gets the filename of the disk file associated with this BlockFile
+   /// (can be empty -- some BlockFiles, like SilentBlockFile, correspond to
+   ///  no file on disk)
    virtual wxFileName GetFileName();
    virtual void SetFileName(wxFileName &name);
 

src/DirManager.cpp

@@ -973,6 +973,11 @@ bool DirManager::HandleXMLTag(const wxChar *tag, const wxChar **attrs)
 
 bool DirManager::MoveToNewProjectDirectory(BlockFile *f)
 {
+   // Check that this BlockFile corresponds to a file on disk
+   if (f->GetFileName().GetName().IsEmpty()) {
+      return true;
+   }
+
    wxFileName newFileName;
    wxFileName oldFileName=f->GetFileName();
    AssignFile(newFileName,f->GetFileName().GetFullName(),FALSE); 
@@ -1025,6 +1030,10 @@ bool DirManager::MoveToNewProjectDirectory(BlockFile *f)
 
 bool DirManager::CopyToNewProjectDirectory(BlockFile *f)
 {
+   // Check that this BlockFile corresponds to a file on disk
+   if (f->GetFileName().GetName().IsEmpty()) {
+      return true;
+   }
    wxFileName newFileName;
    wxFileName oldFileName=f->GetFileName();
    AssignFile(newFileName,f->GetFileName().GetFullName(),FALSE);