lua: fix crash when a lua function doesn't return a value and a string was expected (closes #718)

ChangeLog.asciidoc

@@ -37,6 +37,8 @@ https://weechat.org/files/releasenotes/ReleaseNotes-devel.html[release notes]
 * irc: add option irc.network.sasl_fail_unavailable (issue #600, issue #697)
 * irc: add multiple targets and support of "-server" in command /ctcp
   (issue #204, issue #493)
+* lua: fix crash when a lua function doesn't return a value and a string was
+  expected (issue #718)
 * ruby: add detection of Ruby 2.3 (issue #698)
 * trigger: add "recover" in default triggers cmd_pass/msg_auth, and "regain"
   in default trigger "msg_auth" (issue #511)

src/plugins/lua/weechat-lua.c

@@ -204,7 +204,19 @@ weechat_lua_exec (struct t_plugin_script *script, int ret_type,
     {
         if (ret_type == WEECHAT_SCRIPT_EXEC_STRING)
         {
-            ret_value = strdup ((char *) lua_tostring (lua_current_interpreter, -1));
+            ret_value = (char *) lua_tostring (lua_current_interpreter, -1);
+            if (ret_value)
+            {
+                ret_value = strdup (ret_value);
+            }
+            else
+            {
+                weechat_printf (NULL,
+                                weechat_gettext ("%s%s: function \"%s\" must "
+                                                 "return a valid value"),
+                                weechat_prefix ("error"), LUA_PLUGIN_NAME,
+                                function);
+            }
         }
         else if (ret_type == WEECHAT_SCRIPT_EXEC_INT)
         {