Avoid calling scm_i_string_start_writing if no chars will be mutated

Avoid calling scm_i_string_start_writing if no chars will be mutated

* libguile/srfi-13.c (scm_string_copy_x, scm_substring_fill_x,
  string_upcase_x, string_downcase_x, string_titlecase_x,
  string_reverse_x, scm_string_xcopy_x): Avoid calling
  `scm_i_string_start_writing' if the range of indices to be modified is
  empty.  This avoids the error that would be raised by
  `scm_i_string_start_writing' if the string is not mutable.
  Thanks to Bruce Korb <bkorb@gnu.org> for reporting this problem
  and suggesting the fix.