cryptodisk: Improve handling of partition name in cryptomount password prompt

cryptodisk: Improve handling of partition name in cryptomount password prompt

Call grub_partition_get_name() unconditionally to initialize the part
variable. Then part will only be NULL when grub_partition_get_name() errors.
Note that when source->partition is NULL, then grub_partition_get_name()
returns an allocated empty string. So no comma or partition will be printed,
as desired.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>