ISAC_main_inst initialized to NULL to avoid potentially garbage pointer passed to WebRtcIsacfix_EncoderInit

BUG=1211
TESTED=local build on Windows.  Failed previously with vs2012.  With this change kenny.cc builds.
Review URL: https://webrtc-codereview.appspot.com/984004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3277 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/codecs/isac/fix/test/kenny.cc

@@ -138,7 +138,7 @@ int main(int argc, char* argv[])
   WebRtc_Word16 lostFrame;
   float scale = (float)0.7;
   /* only one structure used for ISAC encoder */
-  ISACFIX_MainStruct *ISAC_main_inst;
+  ISACFIX_MainStruct *ISAC_main_inst = NULL;
 
   /* For fault test 10, garbage data */
   FILE *seedfile;