patch 8.1.0613: when executing an insecure function the secure flag is stuck

Problem:    When executing an insecure function the secure flag is stuck.
            (Gabriel Barta)
Solution:   Restore "secure" instead of decrementing it. (closes #3705)
diff --git a/src/buffer.c b/src/buffer.c
index bb94069..66068d9 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5519,6 +5519,7 @@
 
 	    if (*s != NUL)		/* skip over an empty "::" */
 	    {
+		int secure_save = secure;
 #ifdef FEAT_EVAL
 		save_current_sctx = current_sctx;
 		current_sctx.sc_sid = SID_MODELINE;
@@ -5530,7 +5531,7 @@
 
 		retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);
 
-		--secure;
+		secure = secure_save;
 #ifdef FEAT_EVAL
 		current_sctx = save_current_sctx;
 #endif