markov: fixed severe bug in Shift()

modules/markov.go

@@ -129,8 +129,12 @@ func (p MarkovPrefix) String() string {
 
 // Shift removes the first word from the Prefix and appends the given word.
 func (p MarkovPrefix) Shift(word string) {
+	if len(p) < *markovPrefixLen {
+		p = append(p, word)
+	} else {
 		copy(p, p[1:])
 		p[len(p)-1] = word
+	}
 }
 
 // MarkovChain contains a map ("chain") of prefixes to a list of suffixes.