libxl/dm_depriv: Fix non-debug build

libxl/dm_depriv: Fix non-debug build

Apparently older versions of gcc, when building with debug=n, the
uninitialized variable logic gets confused.  Distros on which a
non-debug build will fail include:
- Centos 7
- Debian Jessie
- Ubuntu Trusty

It seems to be one particular path confusing the logic; so just set it
on that path to keep the compiler happy, while still catching other
potential paths where it might be unset.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>