GB28181 code crashed in ffmpeg after commit "RTC: Use FFmpeg to transcode aac to opus" <d5a0ad3dd8>. (#2057)

Change the size from 64K to 256K.

trunk/src/app/srs_app_gb28181.cpp

@@ -1278,6 +1278,8 @@ SrsGb28181RtmpMuxer::SrsGb28181RtmpMuxer(SrsGb28181Manger* c, std::string id, bo
 
     pprint = SrsPithyPrint::create_caster();
     trd = new SrsSTCoroutine("gb28181rtmpmuxer", this);
+    //change stack size to 256K, fix crash when call FFMpeg
+    ((SrsSTCoroutine*)trd)->set_stack_size(1 << 18);
     
     sdk = NULL;
     vjitter = new SrsRtspJitter();