Fix incorrect header size

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.14)
 
-project(libXISF VERSION 0.2.8 LANGUAGES CXX C
+project(libXISF VERSION 0.2.10 LANGUAGES CXX C
     HOMEPAGE_URL https://gitea.nouspiro.space/nou/libXISF
     DESCRIPTION "LibXISF is C++ library that can read and write XISF files produced by PixInsight.")
 

libxisf.cpp

@@ -957,7 +957,7 @@ void XISFWriterPrivate::writeHeader()
         offset += image._dataBlock.data.size();
     }
 
-    uint32_t headerSize = size - sizeof(signature);
+    uint32_t headerSize = header.size() - sizeof(signature);
     header.resize(size, 0);
     header.replace(8, sizeof(uint32_t), (const char*)&headerSize, sizeof(uint32_t));