s390/numa: correct early_param handling

When command line options are used without specifying values
(e.g. "emu_size" instead of "emu_size="), the value is NULL. Check that
before performing string operations and further processing.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/numa/numa.c

@@ -158,6 +158,8 @@ early_param("numa_debug", parse_debug);
 
 static int __init parse_numa(char *parm)
 {
+	if (!parm)
+		return 1;
 	if (strcmp(parm, numa_mode_plain.name) == 0)
 		mode = &numa_mode_plain;
 #ifdef CONFIG_NUMA_EMU