rpc: avoid uninitialized memory use

Spotted by Coverity.  Gnutls documents that buffer must be NULL
if gnutls_x509_crt_get_key_purpose_oid is to be used to determine
the correct size needed for allocating a buffer.

* src/rpc/virnettlscontext.c
(virNetTLSContextCheckCertKeyPurpose): Initialize buffer.

src/rpc/virnettlscontext.c

@@ -264,7 +264,7 @@ static int virNetTLSContextCheckCertKeyPurpose(gnutls_x509_crt_t cert,
     int i;
     unsigned int purposeCritical;
     unsigned int critical;
-    char *buffer;
+    char *buffer = NULL;
     size_t size;
     bool allowClient = false, allowServer = false;