Correctly handle FLV_CODECID_NELLYMOSER_8HZ_MONO files

Originally committed as revision 11284 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/flvdec.c

@@ -179,6 +179,7 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream, AVStream *vst
             }
             else if(!strcmp(key, "audiosamplerate") && acodec && num_val >= 0) {
                 //some tools, like FLVTool2, write consistently approximate metadata sample rates
+                if (!acodec->sample_rate) {
                 switch((int)num_val) {
                     case 44000: acodec->sample_rate = 44100  ; break;
                     case 22000: acodec->sample_rate = 22050  ; break;
@@ -186,6 +187,7 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream, AVStream *vst
                     case 5000 : acodec->sample_rate = 5512   ; break;
                     default   : acodec->sample_rate = num_val;
                 }
+                }
             }
         }
     }