libuuid: ensure variable is initialized [cppcheck]

libuuid: ensure variable is initialized [cppcheck]

This fix has a little bit of a feel of making a static analyzer to be happy
instead of real progress.  If I read the preprocessor directives correctly
it should be impossible hit uninitialized variable.  Then again if a bug
creeps into these ifdef's in that case it is nice to have robust code that
doesn't immediately go wrong.

    libuuid/src/gen_uuid.c:200:20: error: Uninitialized variable: a [uninitvar]
       memcpy(node_id, a, 6);
                       ^
    libuuid/src/gen_uuid.c:197:8: error: Uninitialized variable: a [uninitvar]
      if (!a[0] && !a[1] && !a[2] && !a[3] && !a[4] && !a[5])
           ^

Signed-off-by: Sami Kerola <kerolasa@iki.fi>