flacdec_close(): Be liberal in what you accept.

Functions like close() which terminate an instance of some subsystem
should always be idempotent, i.e. when calling such a function twice
with the same argument, the second call should be a noop.

However, flacdec_close() violates this rule because it dereferences
its private pointer unconditionally. This patch makes the function
idempotent.