[Fix] mpt/io_read/filedata_base_unseekable.hpp: Avoid integer overflo…

…w in EnsureCacheBuffer() when reading unseekable files are close to uint32_max in size.

git-svn-id: https://source.openmpt.org/svn/openmpt/trunk/OpenMPT@20333 56274372-70c3-4bfc-bfc3-4c3a0b034d27

src/mpt/io_read/filedata_base_unseekable.hpp

@@ -50,7 +50,7 @@ class FileDataUnseekable : public IFileData {
 	};
 
 	void EnsureCacheBuffer(std::size_t requiredbuffersize) const {
-		if (cache.size() >= cachesize + requiredbuffersize) {
+		if (cache.size() - cachesize >= requiredbuffersize) {
 			return;
 		}
 		if (cache.size() == 0) {