patch 8.0.1196: crash when t_RF is not set

Problem:    Crash when t_RF is not set. (Brian Pina)
Solution:   Add t_RF to the list of terminal options. (Hirohito Higashi)
diff --git a/src/option.c b/src/option.c
index 2743f16..baa8d85 100644
--- a/src/option.c
+++ b/src/option.c
@@ -3141,6 +3141,7 @@
     p_term("t_ms", T_MS)
     p_term("t_nd", T_ND)
     p_term("t_op", T_OP)
+    p_term("t_RF", T_RFG)
     p_term("t_RB", T_RBG)
     p_term("t_RC", T_CRC)
     p_term("t_RI", T_CRI)