src: Fix memory leak in virNWFilterBindingDispose

https://bugzilla.redhat.com/show_bug.cgi?id=1603025

Commit b57a9aec neglected to VIR_FREE(binding->filtername) as seen
in the following valgrind report

==6423== 17,328 bytes in 1,083 blocks are definitely lost in loss record 2,275 of 2,297
==6423== at 0x4C29BC3: malloc (vg_replace_malloc.c:299)
==6423== by 0x83B20C9: strdup (in /usr/lib64/libc-2.17.so)
==6423== by 0x533C144: virStrdup (virstring.c:977)
==6423== by 0x54BDD53: virGetNWFilterBinding (datatypes.c:865)
==6423== by 0x318D633C: nwfilterBindingCreateXML (nwfilter_driver.c:767)
==6423== by 0x54F3FC5: virNWFilterBindingCreateXML (libvirt-nwfilter.c:701)
==6423== by 0x539CE29: virDomainConfNWFilterInstantiate (domain_nwfilter.c:116)
==6423== by 0x31E516C2: qemuInterfaceBridgeConnect (qemu_interface.c:589)
==6423== by 0x31D98B56: qemuBuildInterfaceCommandLine (qemu_command.c:8418)
==6423== by 0x31D9F783: qemuBuildNetCommandLine (qemu_command.c:8673)
==6423== by 0x31D9F783: qemuBuildCommandLine (qemu_command.c:10354)
==6423== by 0x31DE355F: qemuProcessLaunch (qemu_process.c:6292)
==6423== by 0x31DE7881: qemuProcessStart (qemu_process.c:6686)

and

==6423== 17,328 bytes in 1,083 blocks are definitely lost in loss record 2,276 of 2,297
==6423== at 0x4C29BC3: malloc (vg_replace_malloc.c:299)
==6423== by 0x83B20C9: strdup (in /usr/lib64/libc-2.17.so)
==6423== by 0x533C144: virStrdup (virstring.c:977)
==6423== by 0x54BDD53: virGetNWFilterBinding (datatypes.c:865)
==6423== by 0x318D641F: nwfilterBindingLookupByPortDev (nwfilter_driver.c:678)
==6423== by 0x54F3B63: virNWFilterBindingLookupByPortDev (libvirt-nwfilter.c:593)
==6423== by 0x539CBC5: virDomainConfNWFilterTeardownImpl.isra.0 (domain_nwfilter.c:136)
==6423== by 0x539CFA5: virDomainConfVMNWFilterTeardown (domain_nwfilter.c:170)
==6423== by 0x31DE5651: qemuProcessStop (qemu_process.c:6912)
==6423== by 0x31E37974: qemuDomainDestroyFlags (qemu_driver.c:2229)
==6423== by 0x54C24BB: virDomainDestroy (libvirt-domain.c:475)
==6423== by 0x1589A2: remoteDispatchDomainDestroy (remote_daemon_dispatch_stubs.h:4827)
==6423== by 0x1589A2: remoteDispatchDomainDestroyHelper (remote_daemon_dispatch_stubs.h:4803)

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>