What is the vulnerability ?

int main (int argc, char *argv[]) { 
     char k[3]; 
     int i=0,j=1; 
     char buffer[50]; 
     strncpy(buffer, argv[1], sizeof(buffer) - 1); 
     buffer[49]='/0'; 
     unsigned char ch='a'; 
     k[0]=1; 
     do{   
        i++;   
        k[i]=ch+i; 
    } while(i<3); 

    return 0; 
}