realloc(): invalid next size Aborted (core dumped)

Question

I am trying to make a huffman coding program. As the first step I'm trying to list the letters in a given paragraph and the amount of the letters. I don't know my logic is right or not but I'm getting a realloc error. I couldn't figure out why. Here is my code:

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

#define fpath1 "reference.txt"

typedef struct huffman{
    char x;
    int xnum;
    struct huffman *left;
    struct huffman *right;
}huff;

void HuffmanTree();

void main(void){
    HuffmanTree();
}

void HuffmanTree(){
    huff *tree;
    int i=0,j,k,flag=0;
    char *x;
    FILE *fp;
    fp=fopen(fpath1,"r");
    x=(char*)malloc(sizeof(char));
    tree=(huff*)malloc(sizeof(huff));
    while(!feof(fp)){
        fscanf(fp,"%c",&x[i]);
        x=(char *)realloc(x,(i+2)*sizeof(char));
        i++;
        printf("%d-",i); //test.
    }
    for(j=0;j<i;j++){
        tree[j].xnum=1;
        for(k=0;k<j;k++){
            if(tree[j].x==x[k]){ //if there is a character x[i] in tree[j] before.
                flag=1;
                tree[j].xnum++; 
                tree=(huff*)realloc(tree,(j+2)*sizeof(huff));
                k=j;
            }
        }
        if(flag==0) tree[j].x=x[k];
        flag=0;
    }

    for(i=0;i<100;i++) printf("%c %d\n",tree[i].x,tree[i].xnum);
}