MFC r316530, r316535

r316530:
Fix clearing geom metadata if DIOCGSECTORSIZE fails

An unhandled error case would result in passing SIZE_MAX to malloc.
While I'm here, remove an unnecessary NULL check before free

Reported by: Coverity
CID: 1017793
Sponsored by: Spectra Logic Corp

r316535:
Fix memory leak in "gpart bootcode"

Also, annotate that gpart_issue never returns

Reported by: Coverity
CID: 1007105
Sponsored by: Spectra Logic Corp