Use xfree() for heap area allocated by xmalloc

[Watson1978]

In buffer.c, it uses xmalloc Ruby API to allocate heap area.

• cool.io/ext/cool.io/buffer.c

  Line 445 in 6f85a2a

  buf = (struct buffer *) xmalloc(sizeof(struct buffer));

• cool.io/ext/cool.io/buffer.c

  Line 509 in 6f85a2a

  node = (struct buffer_node *) xmalloc(sizeof(struct buffer_node) + buf->node_size);

To release the areas, it should use xfree instead of free.

Types of Changes

• Bug fix.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.