Ticket #884: Fixed codecs (that use pjmedia VAD) to avoid generating keep-alive frames/payload when PJMEDIA_CODEC_MAX_SILENCE_PERIOD is set to -1.

git-svn-id: https://svn.pjsip.org/repos/pjproject/trunk@2760 74dad513-b988-da41-8d7b-12977e46ad98
diff --git a/pjmedia/src/pjmedia-codec/gsm.c b/pjmedia/src/pjmedia-codec/gsm.c
index 2ecd03a..965c936 100644
--- a/pjmedia/src/pjmedia-codec/gsm.c
+++ b/pjmedia/src/pjmedia-codec/gsm.c
@@ -542,8 +542,8 @@
 						(input->size >> 1),
 						NULL);
 	if (is_silence &&
-	    PJMEDIA_CODEC_MAX_SILENCE_PERIOD != -1 &&
-	    silence_duration < PJMEDIA_CODEC_MAX_SILENCE_PERIOD*8000/1000) 
+	    (PJMEDIA_CODEC_MAX_SILENCE_PERIOD == -1 ||
+	     silence_duration < PJMEDIA_CODEC_MAX_SILENCE_PERIOD*8000/1000))
 	{
 	    output->type = PJMEDIA_FRAME_TYPE_NONE;
 	    output->buf = NULL;