Handle partial characters at the end of the read buffer when loading Lua scripts. Closes #1576.

aegisub/src/auto4_lua_scriptreader.cpp

@@ -50,6 +50,8 @@ namespace Automation4 {
 		size_t out_bytes = sizeof(buf);
 
 		conv->Convert(&in, &in_bytes, &out, &out_bytes);
+		if (in_bytes > 0 && in != in_buf)
+			file.seekg(-(std::streamoff)in_bytes, std::ios_base::cur);
 		*bytes_read = out - buf;
 
 		// Skip the bom