clang: suppress a warning

Initialize variable explicitly.

    ii.c:2909:17: warning: variable 'dcn' is used uninitialized whenever '||'
          condition is true [-Wsometimes-uninitialized]
                if (!actual_chunk_size || !(rc = chunk_new(ctx, ii, &dcn,...
                    ^~~~~~~~~~~~~~~~~~
    ii.c:2911:50: note: uninitialized use occurs here
                  fake_map2(ctx, ii->chunk, &dw, dc, dcn, actual_chunk_size);
                                                     ^~~
    ii.c:2909:17: note: remove the '||' if its condition is always false
                if (!actual_chunk_size || !(rc = chunk_new(ctx, ii, &dcn,...
                    ^~~~~~~~~~~~~~~~~~~~~
    ii.c:2886:30: note: initialize the variable 'dcn' to silence this warning
      uint32_t ds, pseg, scn, dcn;
                                 ^
                                  = 0