added check if custom func already exists

rpnc

@@ -556,6 +556,8 @@ sub defun {
     return;
   }
 
+  if (! exists $custom{$name}) {
+    # no need to check twice and overwriting of custom function must be legal
     if (grep {$name eq $_} keys %commands) {
       print "reserved function name (command)!\n";
       return;
@@ -565,6 +567,7 @@ sub defun {
       print "reserved function name (function)!\n";
       return;
     }
+  }
 
   $custom{$name} = "@tokens";