Fix missing lock calls on virNetTLSContextRef

The virNetTLSContextRef API forgot to acquire/release the lock
while changing ctxt->refs

* src/rpc/virnettlscontext.c: Add lock calls

src/rpc/virnettlscontext.c

@@ -931,7 +931,9 @@ virNetTLSContextPtr virNetTLSContextNewClient(const char *cacert,
 
 void virNetTLSContextRef(virNetTLSContextPtr ctxt)
 {
+    virMutexLock(&ctxt->lock);
     ctxt->refs++;
+    virMutexUnlock(&ctxt->lock);
 }