brain: fix panic on @ at end of message

brain/words.go

@@ -33,6 +33,9 @@ func Tokens(dst []string, msg string) []string {
 			// write for a case that will be uncommon.
 			// In terms of control flow, we can fall through to the next case,
 			// after we skip past the @ itself.
+			if l == len(msg) {
+				break
+			}
 			l++
 			fallthrough
 		case unicode.Is(ln, c):

brain/words_test.go

@@ -53,6 +53,11 @@ func TestWords(t *testing.T) {
 			msg:  "@bocchi ryo",
 			want: s("@bocchi ", "ryo "),
 		},
+		{
+			name: "at-end",
+			msg:  "bocchi@",
+			want: s("bocchi", "@ "),
+		},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {