mem: use slice capacity instead of length, to determine whether to pool buffers or directly allocate them

[PapaCharlie]

Fixes #7631

As the issue states, mem.NewBuffer would not pool buffers with a length below the pooling threshold but whose capacity is actually larger than the pooling threshold. This can lead to buffers being leaked.

RELEASE NOTES:

• mem: use slice capacity instead of length, to determine whether to pool buffers or directly allocate them