[C][Client]Fix data lost when libcurl write-data callback function is called multiple times

Created by: ityuhui

When using libcurl to get the http response, the application program needs to configure the write-data callback function.

       curl_easy_setopt(handle,
                         CURLOPT_WRITEFUNCTION,
                         writeDataCallback);
       curl_easy_setopt(handle,
                         CURLOPT_WRITEDATA,
                         &apiClient->dataReceived);

When libcurl receives data(http response), it will call the callback function (defined in openapi-generator/c-libcurl)

size_t writeDataCallback(void *buffer, size_t size, size_t nmemb, void *userp) {
...
}

to put the data from void *buffer to user's memory location void *userp (&apiClient->dataReceived).

But sometimes (commonly the response data is big) for a http request, the response data will be divided to multiple parts and sent by multiple times, so the callback function writeDataCallback will be called multiple times.

The current writeDataCallback will lose the data the last time received:

size_t writeDataCallback(void *buffer, size_t size, size_t nmemb, void *userp) {
    *(char **) userp = strdup(buffer);    <=== The old userp will be discarded, and userp points to a new memory.
    return size * nmemb;
}

So I changed the apiClient_t and writeDataCallback

typedef struct apiClient_t {
...
    long dataReceivedLen;
...
} apiClient_t;

       curl_easy_setopt(handle,
                         CURLOPT_WRITEDATA,
                         apiClient);

size_t writeDataCallback(void *buffer, size_t size, size_t nmemb, void *userp) {
    size_t size_this_time = nmemb * size;
    apiClient_t *apiClient = (apiClient_t *)userp;
    apiClient->dataReceived = (char *)realloc( apiClient->dataReceived, apiClient->dataReceivedLen + size_this_time + 1);   <== now the previous data will be reserved.
    memcpy(apiClient->dataReceived + apiClient->dataReceivedLen, buffer, size_this_time);
    apiClient->dataReceivedLen += size_this_time;
    return size_this_time;
}

Reference: https://stackoverflow.com/questions/3573146/problem-using-libcurl-it-does-not-appear-to-get-the-entire-page

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