fix(agent): nr_execute_handle_autoload needs null check for txn (#1086)

`nr_execute_handle_autoload` wasn't accounting for the possibility of
txn being null which would lead to unpredictable behavior when accessing
it
[here](https://github.com/newrelic/newrelic-php-agent/blob/dev/agent/php_execute.c#L894),
and we would cause corruption when we [set this
value](https://github.com/newrelic/newrelic-php-agent/blob/dev/agent/php_execute.c#L907).

txn can be null in cases where nr_php_txn_end has been called.
such as:
https://github.com/newrelic/newrelic-php-agent/blob/dev/agent/fw_laravel_queue.c#L355-L356

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;