Fix mismerge that crept into r301289.

The conversion of 0 -> NULL required a rebase at some point, as noted
in r301289 when pfg commited it. In that rebase, three lines remained
that had been removed in a prior version of awk, and one of them had a
0 -> NULL change causing a conflict. The conflict should have been
resolved by removing the three lines, but wasn't. This introduces a
regression into f.split3 test which prior to this commit we were
failing, but a pure onetrueawk wasn't. Remove the offending 3 lines.