patch 8.2.0449: Vim9: crash if return type is invalid

Problem:    Vim9: crash if return type is invalid. (Yegappan Lakshmanan)
Solution:   Always return some type, not NULL.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index a595580..dd31092 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -1375,19 +1375,19 @@
 	    emsg(_("E1007: No white space allowed before <"));
 	else
 	    emsg(_("E1008: Missing <type>"));
-	return NULL;
+	return type;
     }
     *arg = skipwhite(*arg + 1);
 
     member_type = parse_type(arg, type_list);
     if (member_type == NULL)
-	return NULL;
+	return type;
 
     *arg = skipwhite(*arg);
     if (**arg != '>')
     {
 	emsg(_("E1009: Missing > after type"));
-	return NULL;
+	return type;
     }
     ++*arg;