Fix -Wmaybe-uninitialized warning in xdr.c

Fix -Wmaybe-uninitialized warning in xdr.c

While we're at fixing build warnings, here's one unnecessary warning
that can be fixed fairly easily.  The SIZE variable is never actually
use uninitialized, but the compiler cannot make that out and thinks
(correctly) that there is a potential for accessing SIZE without
initializing it.  Make this safe by initializing SIZE to 0.

Tested on x86_64.