Allow for redefinition of user defs.

And some protection for inscribe, empty quotes or quotes that do not
have a symbol at the first item are just consumed without affecting the
user defs.

Because the Gperf wordlist is checked before the user defs hash the
inscribe command cannot overwrite the commands defined in the wordlist.

implementations/C/joy.c

@@ -768,18 +768,21 @@ add_user_def(char *name, JoyList body)
 {
 	struct user_def *s;
 	HASH_FIND_STR(user_defs, name, s);
-	if (s) return; /* no overwrite */
+	if (!s) {
 		s = GC_malloc(sizeof *s);
 		s->name = name;
-	s->body = body;
 		HASH_ADD_KEYPTR(hh, user_defs, s->name, strlen(s->name), s);
 	}
+	s->body = body;
+}
 
 
 void
 inscribe(JoyListPtr stack, __attribute__((unused)) JoyListPtr expression)
 {
 	JoyList quote = pop_list(stack);
+	if (!quote) return;
+	if (joySymbol != quote->head->kind) return;
 	add_user_def(quote->head->value.symbol, quote->tail);
 }