tty/serial: 8250_exar: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language extension to the C90 standard, but the preferred mechanism to declare variable-length types such as these ones is a flexible array member[1][2], introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning in case the flexible array does not occur last in the structure, which will help us prevent some kind of undefined behavior bugs from being inadvertenly introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by this change:

"Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

---
 drivers/tty/serial/8250/8250_exar.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 91e9b070d36d..65898ef90801 100644
@@ -127,7 +127,7 @@ struct exar8250 {
 	unsigned int		nr;
 	struct exar8250_board	*board;
 	void __iomem		*virt;
-	int			line[0];
+	int			line[];
 };
 
 static void exar_pm(struct uart_port *port, unsigned int state, unsigned int old)

2.25.0