MtpSource: prevent a crash when dealing with Nexus4 (bgo#722947)

Similar fix to this recent one[1], but which I managed to
reproduce with a Nexus4.

[1] https://git.gnome.org/browse/banshee/commit/?h=stable-2.6&id=4f69e71970a9db3b8e5930fc6af06718cabc11e4