[#2464] IAX: send audio data only if nbSamples greater than zero

nbSamples=0 end up in an arithmetic exception in libiax

sflphone-common/src/iax/iaxvoiplink.cpp

@@ -364,7 +364,7 @@ IAXVoIPLink::sendAudioFromMic (void)
                         _mutexIAX.enterMutex();
 
                         // Make sure the session and the call still exists.
-                        if (currentCall->getSession() && micDataEncoded != NULL) {
+                        if (currentCall->getSession() && (micDataEncoded != NULL) && (nbSample_ > 0)) {
                             if (iax_send_voice (currentCall->getSession(), currentCall->getFormat(), micDataEncoded, compSize, nbSample_) == -1) {
                                 _debug ("IAX: Error sending voice data.\n");
                             }