bpchar comparison function didn't pay attention on postfix whitespace.
bugfix: wow! gpuhashjoin didn't send kern_rowmap and kern_toastbuf were incorrectly assigned on the kernel argument
bugfix: materialization on gpuhashjoin
- t_off was not adjusted by null status
- gpuhashjoin_projection_datum() put values on device only variables also,
  then it caused an over-writen to the next records; oops.