* libguile/strings.c (scm_string_ref): Add proper range checking for the
empty string.
(scm_string_set_x): Likewise.
Reported by Bill Schottstaedt <bil@ccrma.Stanford.EDU>.
* test-suite/tests/strings.test ("string-ref"): New test prefix.
("string-set!")["empty string", "empty string and non-zero index",
"out of range", "negative index", "regular string"]: New tests.
* NEWS: Update.
* libguile/read.c (scm_read_string): Use `scm_i_make_read_only_string ()' to
return a read-only string, as mandated by R5RS. Reported by Bill
Schottstaedt <bil@ccrma.Stanford.EDU>.
* libguile/strings.c (scm_i_make_read_only_string): New function.
(scm_i_shared_substring_read_only): Special-case the empty string
so that the read-only and read-write empty strings are `eq?'. This
optimization is relied on by the `substring/shared' `empty string'
test case in `srfi-13.test'.
* libguile/strings.h (scm_i_make_read_only_string): New declaration.
* test-suite/tests/strings.test ("string-set!")["literal string"]: New test.
* NEWS: Update.
