miniread: Cannot read core-transformers.sls correctly

[okuoku]

Workarounded: e731e8b

Before this patch, it failed with

*** ERROR: unknown mode?? NEXT_QUOTE
    While loading "./check.scm" at line 25
    While loading "/home/oku/repos/yuni/lib-runtime/selfboot/common/run-program.scm" at line 50
    While loading "/home/oku/repos/yuni/lib-runtime/selfboot/gauche/selfboot-entry.scm" at line 132
Stack Trace:
_______________________________________
  0  (error "unknown mode??" mode)
        at "/home/oku/repos/yuni/lib/yuni/miniread/reader.sls":263
  1  (utf8-read (string->utf8 queue))
        at "./check.scm":20

Found-by: Ribbon