patch 8.0.1758: open_line() returns TRUE/FALSE for success/failure

Problem:    open_line() returns TRUE/FALSE for success/failure.
Solution:   Return OK or FAIL.
diff --git a/src/normal.c b/src/normal.c
index 84867b5..be0e75e 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -8491,7 +8491,7 @@
 #ifdef FEAT_COMMENTS
 		    has_format_option(FO_OPEN_COMS) ? OPENLINE_DO_COM :
 #endif
-		    0, 0))
+		    0, 0) == OK)
 	{
 #ifdef FEAT_CONCEAL
 	    if (curwin->w_p_cole > 0 && oldline != curwin->w_cursor.lnum)