mptk: fix exiftool command

scripts/mptk

@@ -34,7 +34,13 @@ proc rate {rating} {
 }
 
 proc current_rating {} {
-	lindex [split [exec exiftool -Comment [file join $::music_directory [current_file]]] " "] end
+	set filename [file join $::music_directory [current_file]]
+
+	if {[catch {set rating [exec -ignorestderr exiftool -Comment $filename 2>/dev/null]}]} {
+		return {}
+	} else {
+		return [lindex [split $rating " "] end]
+	}
 }
 
 proc hl_rating {rating} {