util/grub-install: Fix NULL pointer dereferences

Two grub_device_open() calls does not have associated NULL checks
for returned values. Fix that and appease the Coverity.

Fixes: CID 314583

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

util/grub-install.c

@@ -1784,6 +1784,8 @@ main (int argc, char *argv[])
 	  fill_core_services (core_services);
 
 	  ins_dev = grub_device_open (install_drive);
+	  if (ins_dev == NULL)
+	    grub_util_error ("%s", grub_errmsg);
 
 	  bless (ins_dev, core_services, 0);
 
@@ -1884,6 +1886,8 @@ main (int argc, char *argv[])
 	  fill_core_services(core_services);
 
 	  ins_dev = grub_device_open (install_drive);
+	  if (ins_dev == NULL)
+	    grub_util_error ("%s", grub_errmsg);
 
 	  bless (ins_dev, boot_efi, 1);
 	  if (!removable && update_nvram)