[lib] Validate buffers when ZSTD_c_stable{In,Out}Buffer is set

Adds the validation of the input/output buffers only. They are still
unused.

lib/compress/zstd_compress_internal.h

@@ -291,6 +291,10 @@ struct ZSTD_CCtx_s {
     ZSTD_cStreamStage streamStage;
     U32    frameEnded;
 
+    /* Stable in/out buffer verification */
+    ZSTD_inBuffer expectedInBuffer;
+    size_t expectedOutBufferSize;
+
     /* Dictionary */
     ZSTD_localDict localDict;
     const ZSTD_CDict* cdict;