Fix crash in libphoto2_port usb module

From: Daniel Drake <dsd@gentoo.org>

usb_open() will return NULL in some situations, for example, lack of
permissions. You'll get a crash if you continue working with that device.
Handle that condition appropriately.

This was identified by using the new libusb-compat-0.1 with libusb-1.0 at
https://bugzilla.novell.com/show_bug.cgi?id=432129

The new setup returns errors from usb_open() on a slightly set of
conditions than libusb-0.1 did. However, it doesn't change that this
bug also existed (in different circumstances) with libusb-0.1

Index: libgphoto2-2.4.3/libgphoto2_port/usb/libusb.c
===================================================================
--- libgphoto2-2.4.3.orig/libgphoto2_port/usb/libusb.c
+++ libgphoto2-2.4.3/libgphoto2_port/usb/libusb.c
@@ -780,6 +780,9 @@ gp_port_usb_match_mtp_device(struct usb_
 #endif
 
 	devh = usb_open (dev);
+	if (!devh)
+		return 0;
+
 	/* get string descriptor at 0xEE */
 	ret = usb_get_descriptor (devh, 0x03, 0xee, buf, sizeof(buf));
 	if (ret > 0) gp_log_data("get_MS_OSD",buf, ret);