fix `maybe-uninitialized` when compiling with GCC 7
fix `maybe-uninitialized` when compiling with GCC 7

The compiler complains about the fact that `vx_zero` maybe be used
uninitialized in `decode_Lagrange` and that's right because the default
implementation of `decode_prepare` doesn't initialize `vx_zero` and if
someone overwrite `decode_Lagrange` without overwriting `decode_prepare`
then it will uses an unitialized `vx_zero`!

vrancurel/ntl,vrancurel/ntl,vrancurel/ntl