1_15 Fibonacci strings

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

char* fibstr(int n)
{
        char* first_str;
        char* prev;
        char* current;
        int i = 0, next_string_len;

        prev = malloc(1000);
        strcpy (prev,"a");

        current = malloc(1000);
        strcpy (current,"b");

        if (n == 1)
        return prev;
        else
        if (n == 2)
            return current;
        else {
             for (i=3;i<=n;i++){
             char* next_str;

             next_string_len = strlen(prev) + strlen(current) + 1;//
             next_str = (char* )malloc(next_string_len);
                     memset(next_str,0,next_string_len);

                     strcat(next_str,prev);
                     strcat(next_str,current);

                     free (prev);

             prev = current;
             current = next_str;
             }
        }
        free (prev);

        return current;
}

int main()
{
        unsigned long n;
        scanf("%d", &n);

        char* result;
        result = fibstr(n);
        printf("%s", result);

        free(result);
        return 0;
}