Corrected computation of end block for GPT erasure

xorriso-dd-target/xorriso-dd-target

@@ -15,7 +15,7 @@
 # announces.
 
 xdt_print_version() {
-  echo "xorriso-dd-target , version 1.5.3 , 2020.01.07.123808"
+  echo "xorriso-dd-target , version 1.5.3 , 2020.01.08.204757"
 }
 
 
@@ -225,7 +225,6 @@ xdt_reset_job() {
   xdt_sudo_cmd=
   xdt_have_su_power=
   xdt_end_after_setup=
-  xdt_dev_bytes=
   xdt_dev_end=
   xdt_dd_bs="bs=1M"
   xdt_dd_silent=
@@ -766,6 +765,7 @@ xdt_write_image() {
 
   # Erase possible GPT backup table at end of device
   # if expr can properly divide device size by 512
+  xdt_dev_bytes=$("$xdt_lsblk_cmd" -n -b -o SIZE /dev/"${2}" | head -1)
   if test -n "$xdt_dev_bytes"
   then
     xdt_dev_end=$(expr "$xdt_dev_bytes" / 512 2>/dev/null)