Check usability of named device following a non-existent / invalid one (#756434)

When multiple devices are named on the command line and (after sorting
and removing duplicates) the device following a non-existent or invalid
one is not checked for usability [1].  In most situations this isn't
noticed as the device gets skipped at the "Searching ...  partitions"
step instead.  However as seen in bug 755495 and commit [2]
checking usability matters.

For example (on CentOS 6.5) a large sector disk device can be edited
when it follows a non-existent or invalid device named on the command
line:

    # modprobe scsi_debug dev_size_mb=128 sector_size=4096
    # fgrep scsi_debug /sys/block/*/device/model
    /sys/block/sdd/device/model:scsi_debug

    # ./gpartedbin /dev/does-not-exist /dev/sdd
    ======================
    libparted : 2.1
    ======================
    Could not stat device /dev/does-not-exist - No such file or directory.
    Device /dev/sdd has a logical sector size of 4096.  Not all parts of GNU Parted support this at the moment, and the working code is HIGHLY EXPERIMENTAL.

    /dev/sdd: unrecognised disk label

When erasing a device don't skip confirming the following device is
usable.

[1] Usable device as implemented by useable_device()
    Must not have a large sector size when GParted is built with an old
    version of libparted which doesn't support large sector sizes and
    must be able to read the first sector.

[2] 362b2db331
    Check disks named on the command line are safe to use too (#755495)

Bug 756434 - GParted dumps core when passing non-existent or invalid
             device on the command line

src/GParted_Core.cc

@@ -255,7 +255,7 @@ void GParted_Core::set_devices_thread( std::vector<Device> * pdevices )
 			if ( lp_device == NULL || ! useable_device( lp_device ) )
 			{
 				// Remove this disk device which isn't useable
-				device_paths.erase( device_paths.begin() + t );
+				device_paths.erase( device_paths.begin() + t-- );
 			}
 		}
 	}