Use-after-free with `CParameter::StableInBuffer`

With `CParameter::StableInBuffer(true)` (`experimental` feature), the C
library keeps a pointer to the input of one streaming call and reads from it
in later calls, including `flush_stream` and `end_stream`, which take no
input. An `InBuffer` only borrows its slice for one call, so safe code could
free it in between, and freed memory was compressed into the frame. The C
library behaves this way since zstd 1.5.4, whether bundled or linked from the
system.

The flaw was corrected by refusing to enable the parameter.