Fix memory leak of transcoding when fallback raises

When the fallback function in transcode_loop raises, it will leak the memory
in rb_econv_t. The following script reproduces the leak:

    10.times do
      100_000.times do
        "\ufffd".encode(Encoding::US_ASCII, fallback: proc { raise })
      rescue
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    451196
    889980
    1328508
    1767676
    2206460
    2645372
    3083900
    3522428
    3960956
    4399484

After:

    12508
    12636
    12892
    12892
    13148
    13404
    13532
    13788
    13916
    13916

0 files changed, 0 insertions, 0 deletions