libblkid: Avoid strlen if only first char is checked

libblkid: Avoid strlen if only first char is checked

A strlen() call can lead to out of boundary read access if the
superblock in question has no nul-bytes after the string. This
could be avoided by using strnlen() but the calls in question
merely existed to check if the string length is not 0.

By changing the calls as proposed with this diff, these files are
in sync with other superblock files, which do exactly the same.