Check return for LIBMTP_Open_Raw_Device_Uncached

The LIBMTP_Open_Raw_Device_Uncached function may return NULL if
an error occurs when attempting to open the device. We need to
check for that possibility.

Incenp-bug-id: 6
(This patch will prevent the crash observed in that bug, but it
does not fix the fact that LIBMTP sometimes fails to open the
device.)

src/scheme-libmtp.c

@@ -54,7 +54,9 @@ find_mtp_device(const char *wanted_serial)
     for ( i = 0, device = NULL; i < n && device == NULL; i++ ) {
         char *serial;
 
-        device = LIBMTP_Open_Raw_Device_Uncached(&(raw_devices[i]));
+        if ( ! (device = LIBMTP_Open_Raw_Device_Uncached(&(raw_devices[i]))) )
+            continue;
+
         if ( (serial = LIBMTP_Get_Serialnumber(device)) == NULL ||
                 strcmp(serial, wanted_serial) != 0 ) {
             LIBMTP_Release_Device(device);
@@ -400,7 +402,9 @@ gfsec_scheme_libmtp_get_supports(gfsec_supports_list_t *list)
     for ( i = 0, device = NULL; i < n && device == NULL; i++ ) {
         char *serial, *name;
 
-        device = LIBMTP_Open_Raw_Device_Uncached(&(raw_devices[i]));
+        if ( ! (device = LIBMTP_Open_Raw_Device_Uncached(&(raw_devices[i]))) )
+            continue;
+
         serial = LIBMTP_Get_Serialnumber(device);
         name = LIBMTP_Get_Friendlyname(device);