pxe: Fix crash if 'sysboot' is run without args

Previously, a NULL pointer dereference would occur if the 'sysboot'
command is executed without any arguments.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_pxe.c b/common/cmd_pxe.c
index 4cbb2b1..abf0941 100644
--- a/common/cmd_pxe.c
+++ b/common/cmd_pxe.c
@@ -1648,7 +1648,7 @@
 
 	is_pxe = false;
 
-	if (strstr(argv[1], "-p")) {
+	if (argc > 1 && strstr(argv[1], "-p")) {
 		prompt = 1;
 		argc--;
 		argv++;