term/i386/pc/vga_text: Prevent out-of-bounds writes to VGA text buffer

term/i386/pc/vga_text: Prevent out-of-bounds writes to VGA text buffer

Coordinates passed to screen_write_char() did not have any checks to
ensure they are not out-of-bounds. This adds an if statement to prevent
out-of-bounds writes to the VGA text buffer.

Signed-off-by: Ryan Cohen <rcohenprogramming@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>