Correct buffer length checking

The buffer length checking does not perform correctly if a buffer is
passed in that is exactly the correct length.

src/cn-encoder.c

diff --git a/src/cn-encoder.c b/src/cn-encoder.c
index bdf072d..3182409 100644
--- a/src/cn-encoder.c
+++ b/src/cn-encoder.c
@@ -35,7 +35,7 @@
   ssize_t size;
 } cn_write_state;
 
-#define ensure_writable(sz) if ((ws->offset<0) || (ws->offset + (sz) >= ws->size)) { \
+#define ensure_writable(sz) if ((ws->offset<0) || (ws->offset + (sz) > ws->size)) { \
   ws->offset = -1; \
   return; \
 }