Fix: Don't complain if CAT music files are missing entirely

Just complain if an index into a CAT file that exists is invalid.

src/base_media_base.h

@@ -26,6 +26,7 @@ struct ContentInfo;
 struct MD5File {
 	/** The result of a checksum check */
 	enum ChecksumResult {
+		CR_UNKNOWN,  ///< The file has not been checked yet
 		CR_MATCH,    ///< The file did exist and the md5 checksum did match
 		CR_MISMATCH, ///< The file did exist, just the md5 checksum did not match
 		CR_NO_FILE,  ///< The file did not exist
@@ -34,6 +35,7 @@ struct MD5File {
 	const char *filename;        ///< filename
 	uint8 hash[16];              ///< md5 sum of the file
 	const char *missing_warning; ///< warning when this file is missing
+	ChecksumResult check_result; ///< cached result of md5 check
 
 	ChecksumResult CheckMD5(Subdirectory subdir, size_t max_size) const;
 };