fix(agent): nr_execute_handle_autoload needs null check for txn

nr_execute_handle_autoload wasn't accounting for the possibility of txn being null.

Author: zsistla

agent/php_execute.c

@@ -891,6 +891,14 @@ static void nr_execute_handle_autoload(const char* filename,
     return;
   }
 
+  /* 
+  *  There is a possibility of the txn being NULL if nr_php_end_txn has been called. 
+  *  Verify txn is not null before dereferencing and continuing on.
+  */  
+  if (NULL == NRPRG(txn)) {
+    return;
+  }
+  
   if (NRPRG(txn)->composer_info.autoload_detected) {
     // autoload already handled
     return;