Correctly convert filesize from Kbyte to byte.

`reader.getFileSize()` return the size of the zim in Kbyte in a
`unsigned int` (32 bits). This is ok as it would overflow if the size
of the size is greater than 4294967295 kbytes (so ~4Tbytes).

However, we need to convert the return size into a unsigned 64 bits integer
else, when converting to bytes, we will overflow at 4Gbytes.
Even in `m_size` is a uint64_t.

src/book.cpp

@@ -87,7 +87,7 @@ void Book::update(const kiwix::Reader& reader)
   m_origId = reader.getOrigId();
   m_articleCount = reader.getArticleCount();
   m_mediaCount = reader.getMediaCount();
-  m_size = reader.getFileSize() << 10;
+  m_size = static_cast<uint64_t>(reader.getFileSize()) << 10;
 
   reader.getFavicon(m_favicon, m_faviconMimeType);
 }

src/reader.cpp

@@ -851,7 +851,7 @@ bool Reader::isCorrupted() const
 unsigned int Reader::getFileSize() const
 {
   zim::File* file = this->getZimFileHandler();
-  zim::offset_type size = 0;
+  zim::size_type size = 0;
 
   if (file != NULL) {
     size = file->getFilesize();