Prevent negative indent value in feed_table_block_tag()

Bug-Debian: https://github.com/tats/w3m/issues/88
table_colspan(struct table *t, int row, int col)
{
    int i;
    for (i = col + 1; i <= t->maxcol && (t->tabattr[row][i] & HTT_X); i++) ;
    return i - col;
}

table_colspan(struct table *t, int row, int col)
{
    int i;
    for (i = col + 1; i <= t->maxcol && (t->tabattr[row][i] & HTT_X); i++) ;
    return i - col;
}
