nb/intel/gm45: Prevent null-deref in get_blc_pwm_freq_value()

IF its first call is get_blc_pwm_freq_value(NULL), null dereference
will occur.

Now when the parameter is NULL, it will return the value of the static
blc_pwm_freq directly, so the original behavior is kept.

Signed-off-by: Bill XIE <persmule@hardenedlinux.org>
Change-Id: I32354aa0fe1a3ca725c2031f973ffad0bda81ad5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74179
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/northbridge/intel/gm45/gma.c

@@ -67,7 +67,8 @@ u16 get_blc_pwm_freq_value(const char *edid_ascii_string)
 	int i;
 	int blc_array_len;
 
-	if (blc_pwm_freq > 0)
+	/* Prevent null-deref on strcmp() below */
+	if (blc_pwm_freq > 0 || !edid_ascii_string)
 		return blc_pwm_freq;
 
 	blc_array_len = get_blc_values(&blc_pwm);