Fix warnings: charset.c: win_lbr_chartabsize(): Dead assignment: HI.

Problem    : Dead assignment @ 1037.
Diagnostic : Harmless issue.
Rationale  : `tab_corr` is in effect unused after signaled point.
             Previous code using it after that point was removed at
             24ebb018e28187c61900b1616e4f79fec9d70878.
Resolution : Remove dead assignment. As only one usage remains, remove
             variable and inline the only usage.