c中读取文件时 老是断点到 _CrtMemBlockHeader 求教如何改

Question

代码

struct CharList {
    char *s;
    CharList *next;
}*KeyWord, *SignSymbol, *CalcuSymbol, *AreaSymbol, *VarSymbol;

CharList *L = (CharList*)malloc(sizeof(CharList)), *L1, *L2;
    L2 = L; L->next = NULL;
    f = fopen("D:\\P\\BYYL\\SymbolTable\\MyProgram.txt", "r");
    int i = 0;
    char *in;
for (;; i++)
    {
        while (!(in = (char*)malloc(50 * sizeof(char))));
        fseek(f, 50 * i, 0);
        fread(in, 1, 50, f);
        if (in[0] == '\0') break;
        L->s = (char*)malloc(50 * sizeof(char));
        strcpy(L->s, in);
        free(in);
        L1 = (CharList*)malloc(sizeof(CharList));
        L->next = L1;
        L = L1;
        L->next = NULL;
    }