virStorageTranslateDiskSourcePool: Avoid double free
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1316370
    
    Consider the following disk for a domain:
    
        <disk type='volume' device='cdrom'>
          <driver name='qemu' type='raw'/>
          <auth username='libvirt'>
            <secret type='iscsi' usage='libvirtiscsi'/>
          </auth>
          <source pool='iscsi-secret-pool' volume='unit:0:0:0' mode='direct' startupPolicy='optional'/>
          <target dev='sda' bus='scsi'/>
          <readonly/>
          <address type='drive' controller='0' bus='0' target='0' unit='0'/>
        </disk>
    
    Now, startupPolicy is currently not allowed for iscsi disks, so
    one would expect an error message to be thrown. But what a
    surprise is waiting for users if they try to start up such
    domain:
    
    ==15724== Invalid free() / delete / delete[] / realloc()
    ==15724==    at 0x4C2B1F0: free (vg_replace_malloc.c:473)
    ==15724==    by 0x54B7A69: virFree (viralloc.c:582)
    ==15724==    by 0x552DC90: virStorageAuthDefFree (virstoragefile.c:1549)
    ==15724==    by 0x552F023: virStorageSourceClear (virstoragefile.c:2055)
    ==15724==    by 0x552F054: virStorageSourceFree (virstoragefile.c:2067)
    ==15724==    by 0x55556AA: virDomainDiskDefFree (domain_conf.c:1562)
    ==15724==    by 0x5557ABE: virDomainDefFree (domain_conf.c:2547)
    ==15724==    by 0x1B43CC42: qemuProcessStop (qemu_process.c:5918)
    ==15724==    by 0x1B43BA2E: qemuProcessStart (qemu_process.c:5511)
    ==15724==    by 0x1B48993E: qemuDomainObjStart (qemu_driver.c:7050)
    ==15724==    by 0x1B489B9A: qemuDomainCreateWithFlags (qemu_driver.c:7104)
    ==15724==    by 0x1B489C01: qemuDomainCreate (qemu_driver.c:7122)
    ==15724==  Address 0x21cfbb90 is 0 bytes inside a block of size 48 free'd
    ==15724==    at 0x4C2B1F0: free (vg_replace_malloc.c:473)
    ==15724==    by 0x54B7A69: virFree (viralloc.c:582)
    ==15724==    by 0x552DC90: virStorageAuthDefFree (virstoragefile.c:1549)
    ==15724==    by 0x12D1C8D4: virStorageTranslateDiskSourcePool (storage_driver.c:3475)
    ==15724==    by 0x1B4396E4: qemuProcessPrepareDomain (qemu_process.c:4896)
    ==15724==    by 0x1B43B880: qemuProcessStart (qemu_process.c:5466)
    ==15724==    by 0x1B48993E: qemuDomainObjStart (qemu_driver.c:7050)
    ==15724==    by 0x1B489B9A: qemuDomainCreateWithFlags (qemu_driver.c:7104)
    ==15724==    by 0x1B489C01: qemuDomainCreate (qemu_driver.c:7122)
    ==15724==    by 0x561CA97: virDomainCreate (libvirt-domain.c:6787)
    ==15724==    by 0x12B6FD: remoteDispatchDomainCreate (remote_dispatch.h:4116)
    ==15724==    by 0x12B61A: remoteDispatchDomainCreateHelper (remote_dispatch.h:4092)
    
    The problem is, in virStorageTranslateDiskSourcePool disk
    def->src->auth is freed, but the pointer is not set to NULL. So
    later, when qemuProcessStop starts to free the domain definition,
    virStorageAuthDefFree() tries to free the memory again, instead
    of jumping out immediately.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>