cmd: sysboot: null check filename

Currently if ${bootfile} is unset and sysboot is invoked with no
filename specified then U-Boot will crash will a null-pointer
dereference. Add the missing check and a matching error print.

Fixes: 993c912d30 ("cmd: sysboot: Create a sysboot command dedicated file")
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>

cmd/sysboot.c

@@ -77,6 +77,10 @@ static int do_sysboot(struct cmd_tbl *cmdtp, int flag, int argc,
 
 	if (argc < 6) {
 		filename = env_get("bootfile");
+		if (!filename) {
+			printf("Specify a filename or set the ${bootfile} environment variable\n");
+			return 1;
+		}
 	} else {
 		filename = argv[5];
 		env_set("bootfile", filename);