lua: don't attempt to garbage collect a null value

When not sandboxed, a script can get access to the metatable and call
`.__gc` with an invalid value like nil, causing a NULL pointer dereference
in Suricata.

Ticket: #8248

src/util-lua-dataset.c

@@ -42,8 +42,10 @@ static int LuaDatasetGC(lua_State *luastate)
 {
     SCLogDebug("gc:start");
     struct LuaDataset *s = (struct LuaDataset *)lua_touserdata(luastate, 1);
-    SCLogDebug("deref %s", s->set->name);
-    s->set = NULL;
+    if (s != NULL && s->set != NULL) {
+        SCLogDebug("deref %s", s->set->name);
+        s->set = NULL;
+    }
     SCLogDebug("gc:done");
     return 0;
 }

src/util-lua-flowlib.c

@@ -45,8 +45,10 @@ static int LuaFlowGC(lua_State *luastate)
 {
     SCLogDebug("gc:start");
     struct LuaFlow *s = (struct LuaFlow *)lua_touserdata(luastate, 1);
-    SCLogDebug("flow %p", s->f);
-    s->f = NULL;
+    if (s != NULL) {
+        SCLogDebug("flow %p", s->f);
+        s->f = NULL;
+    }
     SCLogDebug("gc:done");
     return 0;
 }

src/util-lua-packetlib.c

@@ -45,8 +45,10 @@ static int LuaPacketGC(lua_State *luastate)
 {
     SCLogDebug("gc:start");
     struct LuaPacket *s = (struct LuaPacket *)lua_touserdata(luastate, 1);
-    SCLogDebug("packet %p", s->p);
-    s->p = NULL;
+    if (s != NULL) {
+        SCLogDebug("packet %p", s->p);
+        s->p = NULL;
+    }
     SCLogDebug("gc:done");
     return 0;
 }