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)