Fixed getLocalFile() condition for x-send-file

Until now, addSendfileHeader() was called even when no x-send-file
headers were set. Even though the method itself doesn't do anything,
a call to getLocalFile() was done and would trigger a full download
when using external storage.

Additionally, the storage resolution code is wrong and always
returns the local storage of the root filesystem, which caused the code to
be run anyway.

This commit fixes both issues.
Merge pull request #5961 from owncloud/core-fixgetfiletempfilecondition

Fixed getLocalFile() condition for x-send-file