Issue #252 - Fix off-by-one error

When decrementing the length to account for newlines, also decrement the code point count.

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 Bug Fixes
 
+- Fixed an off-by-one error in `read-line` where the function erroneously reported an extra character was read from `stdin`. Thanks to wasamasa for the bug report.
 - Fixed a CPS optimization issue where multiple copies of the same lambda are introduced during beta expansion, which causes the optimizer to potentially pick up the wrong value when optimizing-out function calls later on. Thanks to @Chant on Github for providing the report and a test program demonstrating the issue.
 
 ## 0.7.2 - February 15, 2018

runtime.c

@@ -6629,6 +6629,7 @@ object Cyc_io_read_line(void *data, object cont, object port)
       while (len > 0 && (buf[len - 1] == '\n' ||
                          buf[len - 1] == '\r')) {
         len--;
+        num_cp--;
       }
       buf[len] = '\0';
       make_string_noalloc(s, buf, len);