usb/dev-mtp: Fix use of uninitialized values

This fixes:

  hw/usb/dev-mtp.c:971:5: warning: 4th function call argument is an uninitialized value
      trace_usb_mtp_op_get_partial_object(s->dev.addr, o->handle, o->path,
                                           c->argv[1], c->argv[2]);
                                                       ^~~~~~~~~~
and:

  hw/usb/dev-mtp.c:981:12: warning: Assigned value is garbage or undefined
      offset = c->argv[1];
               ^ ~~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180604151421.23385-3-f4bug@amsat.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 62713a2e50f653162387451034f1a2490e87be88)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/usb/dev-mtp.c

@@ -965,12 +965,16 @@ static MTPData *usb_mtp_get_object(MTPState *s, MTPControl *c,
 static MTPData *usb_mtp_get_partial_object(MTPState *s, MTPControl *c,
                                            MTPObject *o)
 {
-    MTPData *d = usb_mtp_data_alloc(c);
+    MTPData *d;
     off_t offset;
 
+    if (c->argc <= 2) {
+        return NULL;
+    }
     trace_usb_mtp_op_get_partial_object(s->dev.addr, o->handle, o->path,
                                         c->argv[1], c->argv[2]);
 
+    d = usb_mtp_data_alloc(c);
     d->fd = open(o->path, O_RDONLY);
     if (d->fd == -1) {
         usb_mtp_data_free(d);