Use xfree() for heap area allocated by xmalloc

[Watson1978]

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

• https://github.com/socketry/cool.io/blob/6f85a2a104488e5c7cb128b9a83058d28ba16d37/ext/cool.io/buffer.c#L445
• https://github.com/socketry/cool.io/blob/6f85a2a104488e5c7cb128b9a83058d28ba16d37/ext/cool.io/buffer.c#L509

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

Types of Changes

• Bug fix.

Contribution

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