mux{edit,internal}: fix leaks on error

several calls to ChunkSetHead() were unchecked, causing the chunk to
leak should the call fail due to OOM

Tested:
for i in `seq 1 1125`; do
  export MALLOC_FAIL_AT=$i
  ./examples/gif2webp gif_file
  ./examples/gif2webp -mixed gif_file
done
for i in `seq 1 171`; do
  export MALLOC_FAIL_AT=$i
  ./examples/img2webp jpeg_file -o /dev/null
  ./examples/img2webp -mixed jpeg_file -o /dev/null
done

Change-Id: I479bc487f61b493e5ce033872d353007055c172a

src/mux/muxedit.c

@@ -70,6 +70,7 @@ void WebPMuxDelete(WebPMux* mux) {
     err = ChunkAssignData(&chunk, data, copy_data, tag);                       \
     if (err == WEBP_MUX_OK) {                                                  \
       err = ChunkSetHead(&chunk, (LIST));                                      \
+      if (err != WEBP_MUX_OK) ChunkRelease(&chunk);                            \
     }                                                                          \
     return err;                                                                \
   }