Adding a file_exists() check before unlinking cache files.

This prevents issues where two concurrent requests could
be clearing the same cache files.
Fixes #1936

lib/Cake/Cache/Engine/FileEngine.php

@@ -240,7 +240,9 @@ public function clear($check) {
 			}
 			$path = $this->_File->getRealPath();
 			$this->_File = null;
-			unlink($path);
+			if (file_exists($path)) {
+				unlink($path);
+			}
 		}
 		$dir->close();
 		return true;