Fix memory leak in String#tr and String#tr_s

rb_enc_codepoint_len could raise, which would cause the memory in buf
to leak.

For example:

    str1 = "\xE0\xA0\xA1#{" " * 100}".force_encoding("EUC-JP")
    str2 = ""
    str3 = "a".force_encoding("Windows-31J")

    10.times do
      1_000_000.times do
        str1.tr_s(str2, str3)
      rescue
      end

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

Before:

    17536
    22752
    28032
    33312
    38688
    43968
    49200
    54432
    59744
    64992

After:

    12176
    12352
    12352
    12448
    12448
    12448
    12448
    12448
    12448
    12448

0 files changed, 0 insertions, 0 deletions