util/broadcom: Add two more NULL checks

Change-Id: I088730fd87dd39fa2c36a06c5770fad05a5808b0
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Found-by: Coverity Scan #1323511, #1323512
Reviewed-on: https://review.coreboot.org/17882
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

util/broadcom/secimage/sbi.c

@@ -109,6 +109,16 @@ int CreateSecureBootImage(int ac, char **av)
 		--ac, ++av;
 	}
 
+	if (!bl) {
+		puts("-bl not set");
+		return -1;
+	}
+
+	if (!privkey) {
+		puts("-hmac not set");
+		return -1;
+	}
+
 	if (stat(bl, &file_stat) == -1) {
 		puts("Can't stat bl");
 		return -1;