Auto & in ranged for loop of C++11 not working

I was trying to use auto & in ranged for loop to modify the value. But I was so shocked that it did not work "ALL THE TIME". I attach my original code below. It simply find element 0 in a matrix and set the corresponding row and column to all zeros.

    #include<iostream>
    #include<vector>
    using namespace std;

    void zerolify(vector<vector<int>>& m)
    {
        int row=m.size();
        int col=col>0?m[0].size():0;

        vector<int> r(row,0);
        vector<int> c(col,0);

        for(int i=0;i<row;i++)
        {
            for(int j=0;j<col;j++)
            {
                if(m[i][j]==0)
                {
                    r[i]=1;
                    c[j]=1;
                }
            }
        }

        for(int i=0;i<row;i++)
        {
            if(r[i])
            {
                //cout<<"row: "<<i<<endl;
                for(auto &e : m[i])
                {
                    e=0;
                }
                //for(int j=0;j<col;j++)
                //{
                //  m[i][j]=0;
                //}
            }
        }
        for(int i=0;i<col;i++)
        {
            if(c[i])
            {
                for(int j=0;j<row;j++)
                {
                    m[j][i]=0;
                }
            }
        }
    }
    void printMatrix(vector<vector<int>> m)
    {
        for(auto i:m)
        {
            for(auto j:i)
            {
                cout<<j<<" ";
            }
            cout<<endl;
        }
    }
    int main(int argn, char** argv)
    {
        vector<vector<int>> m(5,vector<int>(5));
        m[0]={1,2,3,0,4};
        m[1]={1,1,3,5,4};
        m[2]={1,2,3,0,4};
        m[3]={1,2,3,5,4};
        m[4]={1,2,3,5,4};
        cout<<"Original matrix:"<<endl;
        printMatrix(m);
        zerolify(m);
        cout<<"Zerolify"<<endl;
        printMatrix(m);
        return 0;
   }

I am using Mac OS 10.12.2 with Apple LLVM version 8.0.0 (clang-800.0.42.1) compiler. Here is my results: My command line output

You can see that the first time it did not work leaving the matrix unchanged. The second time it worked. I tried also auto &&, it gave me the same 'uncertain' behavior. But for loop with direct indexing work all the time (commented out in the code). Currently, I could not replicate the problem in any other case, however the logics here is already simple enough.

Could it be compiler problem ? Can anyone run the code and see if the problem can be replicated ?