At the end of every tbl(7) cell, clear the \z state.

At the end of every tbl(7) cell, clear the \z state.
This is needed because the TERMP_MULTICOL mode is designed such
that term_tbl() buffers all the cells of the table row before the
normal reset logic near the end of term_flushln() can be reached.

This fixes an assertion failure triggered by \z near the end
of a table cell, found by tb@ using afl(1).