x86: make "apic" an early_param() on 32-bit, NULL check

Cyrill Gorcunov observed:

> you turned it into early_param so now it's NULL injecting vulnerabled.
> Could you please add checking for NULL str param?

fix that.

Also, change the name of 'str' into 'arg', to make it more apparent
that this is an optional argument that can be NULL, not a string
parameter that is empty when unset.
Reported-by: NCyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: NRene Herman <rene.herman@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/apic_32.c

@@ -1720,12 +1720,16 @@ static int __init parse_lapic_timer_c2_ok(char *arg)
 }
 early_param("lapic_timer_c2_ok", parse_lapic_timer_c2_ok);
 
-static int __init apic_set_verbosity(char *str)
+static int __init apic_set_verbosity(char *arg)
 {
-	if (strcmp("debug", str) == 0)
+	if (!arg)
+		return -EINVAL;
+
+	if (strcmp(arg, "debug") == 0)
 		apic_verbosity = APIC_DEBUG;
-	else if (strcmp("verbose", str) == 0)
+	else if (strcmp(arg, "verbose") == 0)
 		apic_verbosity = APIC_VERBOSE;
+
 	return 0;
 }
 early_param("apic", apic_set_verbosity);