showfiltfmts: destroy filter context before exit

Originally committed as revision 24722 to svn://svn.ffmpeg.org/ffmpeg/trunk

tools/lavfi-showfiltfmts.c

@@ -93,6 +93,7 @@ int main(int argc, char **argv)
                    av_pix_fmt_descriptors[fmts->formats[j]].name);
     }
 
+    avfilter_destroy(filter_ctx);
     fflush(stdout);
     return 0;
 }