patch 8.2.1423: Vim9: find global function when looking for script-local

Problem:    Vim9: find global function when looking for script-local.
Solution:   Don't strip prefix if name starts with "s:". (closes #6688)
diff --git a/src/userfunc.c b/src/userfunc.c
index f38c978f..9ab5ffe 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -2132,8 +2132,8 @@
 		char_u *p = untrans_function_name(rfname);
 
 		// If using Vim9 script try not local to the script.
-		// TODO: should not do this if the name started with "s:".
-		if (p != NULL)
+		// Don't do this if the name starts with "s:".
+		if (p != NULL && (funcname[0] != 's' || funcname[1] != ':'))
 		    fp = find_func(p, is_global, NULL);
 	    }