emmc: add check "name" sysattr is not NULL
emmc: add check "name" sysattr is not NULL

At this point it has to be present, but better have the check for
correctness sake.
Follow up of
https://github.com/fwupd/fwupd/commit/64fa633e3029ff619a2de86e85e7fbb60cc40225

Change-Id: I51b5995e25dc4dc4ec63b90af271cd90650d184d

hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd