Fix write-beyond-end-of-string error in the conversion to R6RS string escapes.

Fix write-beyond-end-of-string error in the conversion to R6RS string escapes.

Reported by Mike Gran <spk121@yahoo.com>.

* libguile/strings.c (scm_i_unistring_escapes_to_guile_escapes,
  scm_i_unistring_escapes_to_r6rs_escapes): Augment comments.
  (scm_to_stringn): When `handler ==
  SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE && SCM_R6RS_ESCAPES_P', realloc
  BUF so that it's large enough for the worst case.

* libguile/print.c (display_character): When `result != NULL && strategy
  == SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE && SCM_R6RS_ESCAPES_P', make
  LOCALE_ENCODED large enough to hold an R6RS escape.