patch 8.2.5107: some callers of rettv_list_alloc() check for not OK

Problem:    Some callers of rettv_list_alloc() check for not OK. (Christ van
            Willegen)
Solution:   Use "==" instead of "!=" when checking the return value.
diff --git a/src/textprop.c b/src/textprop.c
index 9d3487f..9198665 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -1076,7 +1076,7 @@
 		|| check_for_opt_dict_arg(argvars, 1) == FAIL))
 	return;
 
-    if (rettv_list_alloc(rettv) != OK)
+    if (rettv_list_alloc(rettv) == FAIL)
 	return;
 
     // default: get text properties on current line