rtc: gamecube: Check the return value of ioremap()

The function ioremap() in gamecube_rtc_read_offset_from_sram() can fail
and return NULL, which is dereferenced without checking, leading to a
NULL pointer dereference.

Add a check for the return value of ioremap() and return -ENOMEM on
failure.

Fixes: 8655940 ("rtc: gamecube: Add a RTC driver for the GameCube, Wii and Wii U")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Reviewed-by: Link Mauve <kernel@linkmauve.fr>
Link: https://patch.msgid.link/20251126080625.1752-1-vulab@iscas.ac.cn
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: ISCAS-Vulab

drivers/rtc/rtc-gamecube.c

@@ -242,6 +242,10 @@ static int gamecube_rtc_read_offset_from_sram(struct priv *d)
 	}
 
 	hw_srnprot = ioremap(res.start, resource_size(&res));
+	if (!hw_srnprot) {
+		pr_err("failed to ioremap hw_srnprot\n");
+		return -ENOMEM;
+	}
 	old = ioread32be(hw_srnprot);
 
 	/* TODO: figure out why we use this magic constant.  I obtained it by