patch 8.2.4472: Coverity warns for use of a freed function name

Problem:    Coverity warns for use of a freed function name.
Solution:   Only check an autoload name when is prefixed.
diff --git a/src/userfunc.c b/src/userfunc.c
index ed2fb86..03b310d 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -4624,7 +4624,7 @@
 		{
 		    char_u *prefixed = may_prefix_autoload(name);
 
-		    if (prefixed != NULL)
+		    if (prefixed != NULL && prefixed != name)
 		    {
 			v = find_var(prefixed, &ht, TRUE);
 			if (v != NULL)