util: fix the VIR_STRDUP when src is NULL

util: fix the VIR_STRDUP when src is NULL

When src is NULL, VIR_STRDUP will return 0 directly.
This patch will set dest to NULL before VIR_STRDUP return.

Example:
[root@yds-pc libvirt]# virsh
Welcome to virsh, the virtualization interactive terminal.

Type: 'help' for help with commands
'quit' to quit

virsh # connect
error: Failed to connect to the hypervisor
error: internal error Unable to parse URI �N�*

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Signed-off-by: Eric Blake <eblake@redhat.com>