MFC r298671,r298672:

MFC r298671,r298672:

r298671 (by cem):

g_part_bsd64: Check for valid on-disk npartitions value

This value is u32 on disk, but assigned to an int in memory.  After we do the
implicit conversion via assignment, check that the result is at least one[1]
(non-negative[2]).

1. The subsequent for-loop iterates from gpt_entries minus one, down, until
   reaching zero.  A negative or zero initial index results in undefined signed
   integer overflow.
2. It is also used to index into arrays later.

In practice, we expected non-malicious disks to contain small positive values.

CID:		1223202

r298672 (by cem):

g_part_bsd64: Delete duplicate/dead code

RAW_PART is handled earlier in the loop.

CID:		1223201

git-svn-id: svn://svn.freebsd.org/base/stable/10@299629 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f