Fix memory leak in virNetSocketNewConnectUNIX

Fix memory leak in virNetSocketNewConnectUNIX

==26726==    by 0x673CD67: __vasprintf_chk (vasprintf_chk.c:80)
==26726==    by 0x5673605: UnknownInlinedFun (stdio2.h:210)
==26726==    by 0x5673605: virVasprintfInternal (virstring.c:476)
==26726==    by 0x56736EE: virAsprintfInternal (virstring.c:497)
==26726==    by 0x5680C37: virGetUserRuntimeDirectory (virutil.c:866)
==26726==    by 0x5783A89: virNetSocketNewConnectUNIX (virnetsocket.c:572)
==26726==    by 0x57751AF: virNetClientNewUNIX (virnetclient.c:344)
==26726==    by 0x57689B3: doRemoteOpen (remote_driver.c:895)
==26726==    by 0x5769F8E: remoteConnectOpen (remote_driver.c:1195)
==26726==    by 0x57092DF: do_open (libvirt.c:1189)
==26726==    by 0x570A7BF: virConnectOpenAuth (libvirt.c:1341)

https://bugzilla.redhat.com/show_bug.cgi?id=1215042
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>