Make sure MercurialProcessor fails gracefully

src/Monolog/Processor/MercurialProcessor.php

@@ -51,9 +51,13 @@ class MercurialProcessor
         }
 
         $result = explode(' ', trim(`hg id -nb`));
-        return self::$cache = array(
-            'branch' => $result[1],
-            'revision' => $result[2],
-        );
+        if (count($result) >= 3) {
+            return self::$cache = array(
+                'branch' => $result[1],
+                'revision' => $result[2],
+            );
+        }
+
+        return self::$cache = array();
     }
 }