Explicitly make the ByteBuffer content readonly

This does not change any behaviour, but it would
prevent bugs in the future if something tries to write to the buffer.

If fileMode is MEMORY_MAPPED, this would already fail, so
this extra protection only really applies to the MEMORY mode.

Author: spkrka

src/main/java/com/maxmind/db/BufferHolder.java

@@ -21,14 +21,15 @@ final class BufferHolder {
                 final FileChannel channel = file.getChannel()
         ) {
             if (mode == FileMode.MEMORY) {
-                this.buffer = ByteBuffer.wrap(new byte[(int) channel.size()]);
-                if (channel.read(this.buffer) != this.buffer.capacity()) {
+                final ByteBuffer buf = ByteBuffer.wrap(new byte[(int) channel.size()]);
+                if (channel.read(buf) != buf.capacity()) {
                     throw new IOException("Unable to read "
                             + database.getName()
                             + " into memory. Unexpected end of stream.");
                 }
+                this.buffer = buf.asReadOnlyBuffer();
             } else {
-                this.buffer = channel.map(MapMode.READ_ONLY, 0, channel.size());
+                this.buffer = channel.map(MapMode.READ_ONLY, 0, channel.size()).asReadOnlyBuffer();
             }
         }
     }
@@ -50,7 +51,7 @@ final class BufferHolder {
         while (-1 != (br = stream.read(bytes))) {
             baos.write(bytes, 0, br);
         }
-        this.buffer = ByteBuffer.wrap(baos.toByteArray());
+        this.buffer = ByteBuffer.wrap(baos.toByteArray()).asReadOnlyBuffer();
     }
 
     /*