Blinking cursor drawing bug

TextBlinkProc() tries to draw the insertion indicator even
when it is not visible. The result of TkTextCharBbox() is
ignored (it could be -1).

TkTextCharBbox(textPtr, &index, &x, &y, &w, &h);
TkTextRedrawRegion(textPtr, x - textPtr-
>insertWidth / 2, y,
textPtr->insertWidth, h);