GUACAMOLE-1632: Fill null chars before last printable char with spaces

[corentin-soriano]

This selection :

Clipboard content before this code:

/* Pass through start/end coordinates if they are already in the expected
     * order, adjusting only for final character width */
    if (terminal->selection_start_row < terminal->selection_end_row
|| (terminal->selection_start_row == terminal->selection_end_row
&& terminal->selection_start_column < terminal->selection_end_column)) {

*start_row = terminal->selection_start_row;
*start_col = terminal->selection_start_column;
*end_row   = terminal->selection_end_row;
*end_col   = terminal->selection_end_column + terminal->selection_end_width - 1;

}

And now:

    /* Pass through start/end coordinates if they are already in the expected
     * order, adjusting only for final character width */
    if (terminal->selection_start_row < terminal->selection_end_row
        || (terminal->selection_start_row == terminal->selection_end_row
            && terminal->selection_start_column < terminal->selection_end_column)) {

        *start_row = terminal->selection_start_row;
        *start_col = terminal->selection_start_column;
        *end_row   = terminal->selection_end_row;
        *end_col   = terminal->selection_end_column + terminal->selection_end_width - 1;

    }

[necouchman]

Also, this closes #404.