rpc: Fix memleak in virNetMessageEncodeHeader

My latest patch for RPC rework (a2c304f6) introduced a memory leak.
virNetMessageEncodeHeader() is calling VIR_ALLOC_N(msg->buffer, ...)
despite fact, that msg->buffer isn't VIR_FREE()'d on all paths calling
the function. Therefore, rather than injecting free statement switch to
VIR_REALLOC_N().

src/rpc/virnetmessage.c

@@ -219,7 +219,7 @@ int virNetMessageEncodeHeader(virNetMessagePtr msg)
     unsigned int len = 0;
 
     msg->bufferLength = VIR_NET_MESSAGE_MAX + VIR_NET_MESSAGE_LEN_MAX;
-    if (VIR_ALLOC_N(msg->buffer, msg->bufferLength) < 0) {
+    if (VIR_REALLOC_N(msg->buffer, msg->bufferLength) < 0) {
         virReportOOMError();
         goto cleanup;
     }