usb-linux: Ensure devep != 0

So that we don't index endp_table with a negative index.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

usb-linux.c

diff --git a/usb-linux.c b/usb-linux.c
index feb4d36..2e0495e 100644
--- a/usb-linux.c
+++ b/usb-linux.c
@@ -1029,6 +1029,11 @@
             }
 
             devep = descriptors[i + 2];
+            if ((devep & 0x0f) == 0) {
+                fprintf(stderr, "usb-linux: invalid ep descriptor, ep == 0\n");
+                return 1;
+            }
+
             switch (descriptors[i + 3] & 0x3) {
             case 0x00:
                 type = USBDEVFS_URB_TYPE_CONTROL;