s390/startup: correct "dfltcc" option parsing

Currently if just "dfltcc" is passed as a kernel command line option
"val" going to be NULL, this leads to reading at address 0 in
strcmp(val, "off")

Fix that by making sure "val" is not NULL. This does not affect option
handling logic.
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/boot/ipl_parm.c

@@ -230,7 +230,7 @@ void parse_boot_command_line(void)
 		if (!strcmp(param, "vmalloc") && val)
 			vmalloc_size = round_up(memparse(val, NULL), PAGE_SIZE);
 
-		if (!strcmp(param, "dfltcc")) {
+		if (!strcmp(param, "dfltcc") && val) {
 			if (!strcmp(val, "off"))
 				zlib_dfltcc_support = ZLIB_DFLTCC_DISABLED;
 			else if (!strcmp(val, "on"))