fdisk: Fix bad invalid flag 0x00000 warning message

fdisk: Fix bad invalid flag 0x00000 warning message

This splits check_dos_label() and dos_init() off from get_boot() and gets
rid of the invalid flag 0x00000 warning message due to a check for MBR
signs in zeroized buffer:

	memset(MBRbuffer, 0, 512);

	if (what == create_empty_dos)
		goto got_dos_table;
[...]
got_dos_table:
	if (!valid_part_table_flag(MBRbuffer)) {
[...]
	if (!valid_part_table_flag(pe->sectorbuffer))
		fprintf(stderr, _("Warning: invalid flag 0x%04x of partition "
[...]

Signed-off-by: Francesco Cosoleto <cosoleto@gmail.com>