Fix regression introduced by fix for PR 18703.

Fix regression introduced by fix for PR 18703.

When a user object overrides a versioned definition in a shared object,
it's possible to trigger an internal error because the versioned
definition we saw didn't override the unversioned definition. This
was introduced by the fix for PR 18703. This patch fixes the problem
by reordering the tests so that we do not check ret->is_default()
until we're confident that the symbol is in fact versioned.

gold/
	PR gold/18866
	PR gold/18703
	* symtab.cc (Symbol_table): Reorder conditions to avoid internal error.