rpc: avoid memory leak on virNetTLSContextValidCertificate

* src/rpc/virnettlscontext.c: fix memory leak on
  virNetTLSContextValidCertificate.

* Detected in valgrind run:

==25667==
==25667== 6,085 (44 direct, 6,041 indirect) bytes in 1 blocks are definitely
lost in loss record 326 of 351
==25667==    at 0x4005447: calloc (vg_replace_malloc.c:467)
==25667==    by 0x4F2791F3: _asn1_add_node_only (structure.c:53)
==25667==    by 0x4F27997A: _asn1_copy_structure3 (structure.c:421)
==25667==    by 0x4F276A50: _asn1_append_sequence_set (element.c:144)
==25667==    by 0x4F2743FF: asn1_der_decoding (decoding.c:1194)
==25667==    by 0x4F22B9CC: gnutls_x509_crt_import (x509.c:229)
==25667==    by 0x805274B: virNetTLSContextCheckCertificate
(virnettlscontext.c:1009)
==25667==    by 0x804DE32: testTLSSessionInit (virnettlscontexttest.c:693)
==25667==    by 0x804F14D: virtTestRun (testutils.c:140)
==25667==
==25667== 23,188 (88 direct, 23,100 indirect) bytes in 11 blocks are definitely
lost in loss record 346 of 351
==25667==    at 0x4005447: calloc (vg_replace_malloc.c:467)
==25667==    by 0x4F22B841: gnutls_x509_crt_init (x509.c:50)
==25667==    by 0x805272B: virNetTLSContextCheckCertificate
(virnettlscontext.c:1003)
==25667==    by 0x804DDD1: testTLSSessionInit (virnettlscontexttest.c:673)
==25667==    by 0x804F14D: virtTestRun (testutils.c:140)

* How to reproduce?
% cd libvirt && ./configure && make && make -C tests valgrind
or
% valgrind -v --leak-check=full ./tests/virnettlscontexttest
Signed-off-by: NAlex Jia <ajia@redhat.com>

src/rpc/virnettlscontext.c

@@ -1050,6 +1050,7 @@ static int virNetTLSContextValidCertificate(virNetTLSContextPtr ctxt,
                 goto authdeny;
             }
         }
+        gnutls_x509_crt_deinit(cert);
     }
 
 #if 0