Be sure book file is closed before we leave

Move closing of file in Book destructor. This
guarantees us against leaving the file open under
any case and simplifies also Book use.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.cpp

@@ -344,10 +344,21 @@ namespace {
 ////
 
 
+/// Destructor. Be sure file is closed before we leave.
+
+Book::~Book() {
+
+  close();
+}
+
+
 /// Book::open() opens a book file with a given file name
 
 void Book::open(const string& fName) {
 
+  // Close old file before opening the new
+  close();
+
   fileName = fName;
   ifstream::open(fileName.c_str(), ifstream::in | ifstream::binary);
   if (!is_open())
@@ -361,7 +372,6 @@ void Book::open(const string& fName) {
   if (!good())
   {
       cerr << "Failed to open book file " << fileName << endl;
-      close();
       exit(EXIT_FAILURE);
   }
 }
@@ -479,7 +489,6 @@ void Book::read_entry(BookEntry& entry, int idx) {
   if (!good())
   {
       cerr << "Failed to read book entry at index " << idx << endl;
-      close();
       exit(EXIT_FAILURE);
   }
 }