c fprintf adds extra string

Question

I'm trying to make a program that adds the string I want in the position I want in a file. When running the program it must be called like this: ./addString file_name position text

for example if I have a file that contains 123456789 and I add "hello" in position 3 the new file's content is 123hello456789 (position starts counting from 0). The problem comes when I try to add the new string in positions 0 and 9. It adds it correctly but at the end it also adds an extra string like ^G or ^YÍ6 and I don't know why.

Here's my code:

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


main (int argc, char * argv[]){
    if(argc == 4){

            FILE *fd;
            long size, rest;
            char c;

            fd = fopen(argv[1], "r+");

            fseek(fd, 0, SEEK_END);
            size = ftell(fd)-1 ;

            rest = size - atoi(argv[2]);

            char  buffer[rest];
            fseek(fd, atoi(argv[2]), SEEK_SET);

            int i = 0;
            while(i < rest){
                    c=fgetc(fd);
                    buffer[i] = c;
                    i++;

            }
            fseek(fd, atoi(argv[2]), SEEK_SET);

            fprintf(fd, "%s", argv[3]);
            fprintf(fd, "%s", buffer);

            fclose( fd );

     }

}