rpc: don't destroy xdr before creating it in virNetMessageEncodeHeader

rpc: don't destroy xdr before creating it in virNetMessageEncodeHeader

On OOM, xdr_destroy got called even though it wasn't created yet.

Found by coverity:
Error: UNINIT (CWE-457):
    libvirt-0.10.2/src/rpc/virnetmessage.c:214: var_decl: Declaring
    variable "xdr" without initializer.
    libvirt-0.10.2/src/rpc/virnetmessage.c:219: cond_true: Condition
    "virReallocN(&msg->buffer, 1UL /* sizeof (*msg->buffer) */,
    msg->bufferLength) < 0", taking true branch
    libvirt-0.10.2/src/rpc/virnetmessage.c:221: goto: Jumping to label
    "cleanup"
    libvirt-0.10.2/src/rpc/virnetmessage.c:257: label: Reached label
    "cleanup"
    libvirt-0.10.2/src/rpc/virnetmessage.c:258: uninit_use: Using
    uninitialized value "xdr.x_ops".