Fixed count of makernote tag. Fixes bug #615.

src/tiffcomposite.cpp

@@ -688,9 +688,13 @@ namespace Exiv2 {
 
     uint32_t TiffMnEntry::doCount() const
     {
-        // Count of tag Exif.Photo.MakerNote is the size of the Makernote in bytes
+        if (!mn_) {
+            return TiffEntryBase::doCount();
+        }
+        // Count of IFD makernote in tag Exif.Photo.MakerNote is the size of the
+        // Makernote in bytes
         assert(tiffType() == ttUndefined);
-        return size();
+        return mn_->size();
     }
 
     uint32_t TiffArrayEntry::doCount() const