Check for empty buffer when rendering

For empty documents, `->size` is zero so
`renderer.buffer->ptr[renderer.buffer->size - 1]` will cause an out-of-bounds
read. Empty buffers always point to the global `cmark_strbuf__initbuf` buffer
so we read `cmark_strbuf__initbuf[-1]`.

Author: philipturnbull

src/render.c

@@ -171,7 +171,7 @@ char *cmark_render(cmark_node *root, int options, int width,
   }
 
   // ensure final newline
-  if (renderer.buffer->ptr[renderer.buffer->size - 1] != '\n') {
+  if (renderer.buffer->size == 0 || renderer.buffer->ptr[renderer.buffer->size - 1] != '\n') {
     cmark_strbuf_putc(renderer.buffer, '\n');
   }