crypto: lib/mpi - Fix an "Uninitialized scalar variable" issue

The "err" variable may be returned without an initialized value.

Fixes: 8e3a67f2 ("crypto: lib/mpi - Add error checks to extension")
Signed-off-by: Qianqiang Liu <qianqiang.liu@163.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>