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>

grub-core/term/i386/pc/vga_text.c

@@ -63,6 +63,7 @@ static grub_uint8_t cur_color = 0x7;
 static void
 screen_write_char (int x, int y, short c)
 {
+  if (x < COLS && y < ROWS && x >= 0 && y >= 0)
     VGA_TEXT_SCREEN[y * COLS + x] = grub_cpu_to_le16 (c);
 }