secret: Properly handle @def after virSecretObjAdd in driver

secret: Properly handle @def after virSecretObjAdd in driver

Since the virSecretObjListAdd technically consumes @def on success,
the secretDefineXML should set @def = NULL immediately and process
the remaining calls using a new @objDef variable. We can use use
VIR_STEAL_PTR since we know the Add function just stores @def in
obj->def.

Because we steal @def into @objDef, if we jump to restore_backup:
and @backup is set, then we need to ensure the @def would be
free'd properly, so we'll steal it back from @objDef. For the other
condition this fixes a double free of @def if the code had jumped to
@backup == NULL thus calling virSecretObjListRemove without setting
@def = NULL. In this case, the subsequent call to DefFree would
succeed and free @def; however, the call to EndAPI would also
call DefFree because the Unref done would be the last one for
the @obj meaning the obj->def would be used to call DefFree,
but it's already been free'd because @def wasn't managed right
within this error path.

Signed-off-by: John Ferlan <jferlan@redhat.com>