Add assert about initialisation of pblock.
Add assert about initialisation of pblock.

Some analysis tools complain that pblock might be accessed as nullptr in
getauxblock, which is actually not the case.  assert() this and add a
proper argument about why this can never happen as a comment.

namecoin/namecoin-core,domob1812/huntercore,domob1812/huntercore,namecoin/namecoin-core,domob1812/namecore,pataquets/namecoin-core,brandonrobertz/namecoin-core,domob1812/namecore,namecoin/namecoin-core,namecoin/namecore,pataquets/namecoin-core,domob1812/namecore,namecoin/namecore,brandonrobertz/namecoin-core,brandonrobertz/namecoin-core,pataquets/namecoin-core,namecoin/namecoin-core,domob1812/namecore,namecoin/namecore,brandonrobertz/namecoin-core,brandonrobertz/namecoin-core,pataquets/namecoin-core,domob1812/namecore,domob1812/huntercore,pataquets/namecoin-core,namecoin/namecore,namecoin/namecoin-core,domob1812/huntercore,namecoin/namecoin-core,domob1812/huntercore,pataquets/namecoin-core,namecoin/namecore,domob1812/namecore,brandonrobertz/namecoin-core,namecoin/namecore,domob1812/huntercore