MFC r275740: MFV r275548:

Verify that the block pointer is structurally valid, before attempting to
read it in. It can only be invalid in the case of a ZFS bug, but this
change will help identify such bugs in a more transparent way, by
panic'ing with a relevant message, rather than indexing off the end of an
array or something.

Illumos issue:

5349 verify that block pointer is plausible before reading