Fix uninitialized variable in part-id

Fix uninitialized variable in part-id

With gcc 4.5:

part-id.c: In function ‘main’:
part-id.c:186:9: error: ‘partition_number’ may be used uninitialized in this function

This only affected the error path, where we would return an arbitrary value in
get_part_table_device_file()'s out_partition_number (but return a failure
result). No big deal, but let's initialize it for quiescing gcc.