fd,c just Assert()s that lseek() succeeds

I noticed while researching bug #14555 that fd.c contains two separate
cases like

vfdP->seekPos = lseek(vfdP->fd, (off_t) 0, SEEK_CUR);
Assert(vfdP->seekPos != (off_t) -1);

This seems, um, unwise. It might somehow fail to fail in production
builds, because elsewhere it's assumed that -1 means FileUnknownPos,
but it seems like reporting the error would be a better plan.

regards, tom lane