calloc() then realloc() – which values survive? Answer, explanation & discussion

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

int main(void) {
    int *a = calloc(3, sizeof *a);
    if (a == NULL)
        return 1;
    a[0] = 7;
    int *b = realloc(a, 5 * sizeof *a);
    if (b == NULL) {
        free(a);
        return 1;
    }
    b[4] = 9;
    printf("%d %d %d\n", b[0], b[2], b[4]);
    free(b);
    return 0;
}