Heap corruption I don't know where and why it's coming from.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main() {

    char* szBuffer = (char*)calloc(200,1);

    scanf_s("%s", szBuffer,_msize(szBuffer));

    int length = strlen(szBuffer);


    char* szNewBuffer = (char*)calloc(10,1);

    scanf_s("%s", szNewBuffer,_msize(szNewBuffer));
    int length2 = strlen(szNewBuffer);

    if (length + length2 >_msize(szBuffer)-1 ) {
        realloc(szBuffer, length + length2);

        if (szBuffer == NULL) {


            printf ("assignment failure");

        }

        else {
            printf ("Assignment Successful");
            memcpy(szBuffer + length,szNewBuffer,length2);
            puts(szBuffer);



        }
        free(szBuffer);
        free(szNewBuffer);
        return 0;

    }

    printf("%s\n", szBuffer);
    printf("%s\n", szNewBuffer);
    strcpy_s(szBuffer + length,_msize(szBuffer), szNewBuffer);
    printf("%s", szBuffer);

    free(szBuffer);
    free(szNewBuffer);


    return 0;
}