be more cautious with language parsing

Relates to #76

id3v2.go

@@ -363,7 +363,7 @@ func readID3v2Frames(r io.Reader, offset uint, h *id3v2Header) (map[string]inter
 		case name == "COMM" || name == "COM" || name == "USLT" || name == "ULT":
 			t, err := readTextWithDescrFrame(b, true, true) // both lang and enc
 			if err != nil {
-				return nil, err
+				return nil, fmt.Errorf("could not read %q (%q): %v", name, rawName, err)
 			}
 			result[rawName] = t
 

id3v2frames.go

@@ -457,6 +457,9 @@ func readTextWithDescrFrame(b []byte, hasLang bool, encoded bool) (*Comm, error)
 
 	c := &Comm{}
 	if hasLang {
+		if len(b) < 3 {
+			return nil, fmt.Errorf("hasLang set but not enough data for language information")
+		}
 		c.Language = string(b[:3])
 		b = b[3:]
 	}