fix API functions LMS_StartStream() and LMS_StopStream() :

[mbraune]

pls fix wrong behaviour if stream started without creation in src/API/lms7_api.cpp: functions LMS_StartStream(lms_stream_t stream) and
LMS_StopStream(lms_stream_t stream)
return 0 (success) if stream==NULL or stream->handle==0 this should be changed to return -1

[mbraune]

suggested change:

API_EXPORT int CALL_CONV LMS_StartStream(lms_stream_t *stream)
{
    if (stream==nullptr || stream->handle==0)
        return -1;
    return reinterpret_cast<lime::StreamChannel*>(stream->handle)->Start();
}

API_EXPORT int CALL_CONV LMS_StopStream(lms_stream_t *stream)
{
    if (stream==nullptr || stream->handle==0)
        return -1;
    return reinterpret_cast<lime::StreamChannel*>(stream->handle)->Stop();
}