[buffer] Fix buffer size enlargement (harfruzz PR #62)

We were wrongly, comparing with self.len, not the self.info.len
which is the current length of the info/pos arrays. This was
resulting in inadverently shrinking the buffer sometimes, causing
out-of-bounds access and crash, for example in text-rendering-tests
MORX-31 test.

Author: behdad

src/hb/buffer.rs

@@ -1240,7 +1240,7 @@ impl hb_buffer_t {
 
     #[must_use]
     pub fn ensure(&mut self, size: usize) -> bool {
-        if size < self.len {
+        if size <= self.info.len() {
             return true;
         }