powerpc/legacy_serial: Fix UBSAN: array-index-out-of-bounds

UBSAN complains when a pointer is calculated with invalid
'legacy_serial_console' index, allthough the index is verified
before dereferencing the pointer.

Fix it by checking 'legacy_serial_console' validity before
calculating pointers.

Fixes: 0bd3f9e9 ("powerpc/legacy_serial: Use early_ioremap()")
Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210511010712.750096-1-mpe@ellerman.id.au

arch/powerpc/kernel/legacy_serial.c

@@ -356,13 +356,16 @@ static void __init setup_legacy_serial_console(int console)
 
 static int __init ioremap_legacy_serial_console(void)
 {
-	struct legacy_serial_info *info = &legacy_serial_infos[legacy_serial_console];
-	struct plat_serial8250_port *port = &legacy_serial_ports[legacy_serial_console];
+	struct plat_serial8250_port *port;
+	struct legacy_serial_info *info;
 	void __iomem *vaddr;
 
 	if (legacy_serial_console < 0)
 		return 0;
 
+	info = &legacy_serial_infos[legacy_serial_console];
+	port = &legacy_serial_ports[legacy_serial_console];
+
 	if (!info->early_addr)
 		return 0;