Help me in solving DNASTORAGE problem

help
,
1

My issuehttps://discuss.codechef.com/c/bug-reports/326

getting run time error what should i do

My code

#include <stdio.h>
#include <stdlib.h>


int main(void) {
    char str[20];
    int n, m;
    scanf("%d", &n);

    while (n > 0) {
        scanf("%d", &m);
        scanf("%s", str);

        for (int i = 0; i < m; i++) {
            if (str[i] == '0' && str[i + 1] == '0') {
                printf("A");
            } else if (str[i] == '0' && str[i + 1] == '1') {
                printf("T");
            } else if (str[i] == '1' && str[i + 1] == '0') {
                printf("C");
            } else if (str[i] == '1' && str[i + 1] == '1') {
                printf("G");
            }
            i++;
        }

        printf("\n");
        n--;
    }

    return 0;
}

Learning course: Strings using C
Problem Link: CodeChef: Practical coding for everyone