Fix crash reading NTFS usage when there is no /dev/PTN entry (#764658)

On a 64-bit distribution, with an NTFS file system in a partition
without a /dev entry then GParted will crash when attempting to read
the file system usage.  Not having a /dev entry for the partition is
rare and only known to occur for the disk devices used within Fake RAID
(dmraid) arrays, and then only on Ubuntu 12.04 LTS.  Other/newer
distributions do create /dev entries for partitions found on disk
devices within Fake RAID arrays.

Create mirror Fake RAID array:
    # dmraid -f isw -C MyArray --type 1 --disk /dev/sdc,/dev/sdd
    # dmraid -ay

Create NTFS partition on the Fake RAID array.  On refresh GParted
crashes:
    # ./gpartedbin
    (gpartedbin:590): glibmm-ERROR **:
    unhandled exception (type std::exception) in signal handler:
    what: basic_string::assign

Without a /dev/sdc1 device entry the ntfsresize command reports this:
    # ntfsresize --info --force --no-progress-bar /dev/sdc1
    ntfsresize v2015.3.14 (libntfs-3g)
    ERROR(2): Failed to check '/dev/sdc1' mount state: No such file or directory
    Probably /etc/mtab is missing. It's too risky to continue. You might try
    an another Linux distro.

The problem code in ntfs::set_used_sectors():
    145         index = output.find( "Cluster size" );
    146         if ( index == output.npos ||
    147              sscanf( output.substr( index ).c_str(), "Cluster size       : %Ld", &S ) != 1 )
As "Cluster size" did not exist in the output find() returned the not
found token of string::npos [1], which in a 64-bit environment is
represented by 2^64-1 [2].  However it was saved in the variable index
of type unsigned integer, which is only a 32-bit integer, thus
truncating it to 2^32-1.  Therefore the comparison failed and sscanf()
tried to parse the output starting at offset 2^32-1 which resulted in
the crash.

Introduced by commit:
    324d99a1
    Record file system block size where known (#760709)

Fix by following the same pattern of the other comparisons in
ntfs::set_used_sectors() which checks if index is less than the output
length.

References:
[1] std::string::find
    http://www.cplusplus.com/reference/string/string/find/
[2] std::string::npos
    http://www.cplusplus.com/reference/string/string/npos/
(Note that Glib::ustring is derived from std::string in the Standard C++
library and provides a compatible interface).

Bug 764658 - GParted crashes when reading NTFS usage when there is no
             /dev/PTN entry