detect/lua: remove extra NULL check

The fn DetectLuaFree checks if the pointer (DetectLuaData) passed to it
is valid. So, the checks for NULL in the call sites can safely be
removed.

src/detect-lua.c

@@ -482,8 +482,7 @@ static DetectLuaData *DetectLuaParse (DetectEngineCtx *de_ctx, const char *str)
     return lua;
 
 error:
-    if (lua != NULL)
-        DetectLuaFree(de_ctx, lua);
+    DetectLuaFree(de_ctx, lua);
     return NULL;
 }