[C][Client][Clang Static Analyzer] Fix memory leak in apiClient_invoke

[ityuhui]

The 2 problems are found by code static check tool (Clang Static Analyzer)

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
API	Argument with 'nonnull' attribute passed null	src/apiClient.c	apiClient_invoke	275	12
Memory error	Memory leak	src/apiClient.c	apiClient_invoke	257	18

           list_ForEach(listEntry, contentType) {
                if(strstr((char *) listEntry->data,
                          "xml") == NULL)
                {   
                    buffContent =     //<==== The pointer pointing the previous bufContnet memory is lost 
                        malloc(strlen(
                                   "Content-Type: ") + strlen(
                                   (char *)
                                   listEntry->data) +
                               1);     
                    sprintf(buffContent, "%s%s",
                            "Content-Type: ",
                            (char *) listEntry->data);
                    headers = curl_slist_append(headers,
                                                buffContent);


                  // need to free the memory of bufContnet here
               }   
            }   

        if(formParameters != NULL) {
             if(strstr(buffContent,     // <=== buffContent only includes the current content type, it is not enough, we need traverse in list contentType 
                       "application/x-www-form-urlencoded") != NULL)

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@wing328 @zhemant @michelealbano

[wing328]

LGTM