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.
Reported by: Coverity
CID: 1223202
Sponsored by: EMC / Isilon Storage Division
5ad33e776f