patch 8.1.1046: the "secure" variable is used inconsistently

Problem:    the "secure" variable is used inconsistently. (Justin M. Keyes)
Solution:   Set it to one instead of incrementing.
diff --git a/src/buffer.c b/src/buffer.c
index 158c30a..6cfa41f 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5498,7 +5498,7 @@
 		current_sctx.sc_lnum = 0;
 #endif
 		// Make sure no risky things are executed as a side effect.
-		++secure;
+		secure = 1;
 
 		retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);