clear_line(): replace size check with a resize.

Turns out that my assertion that term->cols == line->cols can
sometimes fail, because if the window is shrunk, scrlineptr()
deliberately _doesn't_ shrink the line (so that the columns on the
right can be recovered if the window is then resized larger again). So
clear_line() should _make_ the line the right width, instead of
asserting that it already is.

terminal.c

@@ -2110,7 +2110,7 @@ static void check_selection(Terminal *term, pos from, pos to)
 
 static void clear_line(Terminal *term, termline *line)
 {
-    assert(term->cols == line->cols);
+    resizeline(term, line, term->cols);
     for (int i = 0; i < term->cols; i++)
         copy_termchar(line, i, &term->erase_char);
     line->lattr = LATTR_NORM;