Added missing check for array key in `GelfMessageFormatter`

src/Monolog/Formatter/GelfMessageFormatter.php

@@ -89,7 +89,7 @@ class GelfMessageFormatter extends NormalizerFormatter
             $message->setAdditional($this->contextPrefix . $key, is_scalar($val) ? $val : $this->toJson($val));
         }
 
-        if (null === $message->getFile() && isset($record['context']['exception'])) {
+        if (null === $message->getFile() && isset($record['context']['exception']['file'])) {
             if (preg_match("/^(.+):([0-9]+)$/", $record['context']['exception']['file'], $matches)) {
                 $message->setFile($matches[1]);
                 $message->setLine($matches[2]);