riff.cc: Fail if lseek() fails

riff.cc: Fail if lseek() fails

Fixes Coverity CID 709362: Argument cannot be negative (NEGATIVE_RETURNS)
Function "lseek(this->fd, 0LL, 0)" returns a negative number.
Assigning: signed variable "pos" = "lseek".
...
"pos" is passed to a parameter that cannot be negative.
548                fail_if( lseek( fd, pos, SEEK_SET ) == ( off_t ) - 1 );