patch 8.1.0515: reloading a script gives errors for existing functions

Problem:    Reloading a script gives errors for existing functions.
Solution:   Allow redefining a function once when reloading a script.
diff --git a/src/option.c b/src/option.c
index 3c54ab6..a4a9c97 100644
--- a/src/option.c
+++ b/src/option.c
@@ -415,7 +415,7 @@
     char_u	*def_val[2];	// default values for variable (vi and vim)
 #ifdef FEAT_EVAL
     sctx_T	script_ctx;	// script context where the option was last set
-# define SCTX_INIT , {0, 0}
+# define SCTX_INIT , {0, 0, 0}
 #else
 # define SCTX_INIT
 #endif
@@ -5959,6 +5959,7 @@
 	    else
 	    {
 		script_ctx.sc_sid = set_sid;
+		script_ctx.sc_seq = 0;
 		script_ctx.sc_lnum = 0;
 	    }
 	    set_option_sctx_idx(idx, opt_flags, script_ctx);