add: assert of convolution

[TumoiYorozu]

Add assert for length of input $|a| + |b|$ in convolution_ll and convolution_fft.

https://github.com/atcoder/ac-library/blob/master/document_en/convolution.md?plain=1#L37

The following code should fail on assert

#include <vector>
#include <iostream>
#include <atcoder/convolution>
using namespace std;
using namespace atcoder;


void conv(const int n){
    vector<long long> a(n, 1);
    vector<long long> b(n, 1);

    auto res = convolution<998244353>(a, b);

    for(int i = 0; i < 100; ++i) {
        cout << i << ": " << res[i] << endl;
    }
}


void ll(const int n){
    vector<long long> a(n, 1);
    vector<long long> b(n, 1);

    auto res = convolution_ll(a, b);

    for(int i = 0; i < 100; ++i) {
        cout << i << ": " << res[i] << endl;
    }
}

int main(){
    conv(1 << 23);
    ll(1 << 24);
}

[TumoiYorozu]

static_assert の第2引数を省略すると C++17 が要求されるのですね。コメントを入れればC++11で大丈夫なようなので追加します。
I see that if I omit the second argument to static_assert, C++17 is required. I'll add a comment so that C++11 will be ok.