Set to NULL members that have been freed to prevent crashes

Set to NULL members that have been freed to prevent crashes

Do not crash if virStreamFinish is called after error.

==11000== Invalid read of size 4
==11000==    at 0x373A8099A0: pthread_mutex_lock (pthread_mutex_lock.c:51)
==11000==    by 0x4C7CADE: virMutexLock (threads-pthread.c:85)
==11000==    by 0x4D57C31: virNetClientStreamRaiseError (virnetclientstream.c:203)
==11000==    by 0x4D385E4: remoteStreamFinish (remote_driver.c:3541)
==11000==    by 0x4D182F9: virStreamFinish (libvirt.c:14157)
==11000==    by 0x40FDC4: cmdScreenshot (virsh.c:3075)
==11000==    by 0x42BA40: vshCommandRun (virsh.c:14922)
==11000==    by 0x42ECCA: main (virsh.c:16381)
==11000==  Address 0x59b86c0 is 16 bytes inside a block of size 216 free'd
==11000==    at 0x4A06928: free (vg_replace_malloc.c:427)
==11000==    by 0x4C69E2B: virFree (memory.c:310)
==11000==    by 0x4D57B56: virNetClientStreamFree (virnetclientstream.c:184)
==11000==    by 0x4D3DB7A: remoteDomainScreenshot (remote_client_bodies.h:1812)
==11000==    by 0x4CFD245: virDomainScreenshot (libvirt.c:2903)
==11000==    by 0x40FB73: cmdScreenshot (virsh.c:3029)
==11000==    by 0x42BA40: vshCommandRun (virsh.c:14922)
==11000==    by 0x42ECCA: main (virsh.c:16381)