[lib] Avoid allocating the input buffer when ZSTD_c_stableInBuffer is set

We don't use it when we have a stable input buffer, so don't allocate
it. I had to slightly modify `ZSTD_copyCCtx()` by storing the
`ZSTD_buffered_policy_e` in the `ZSTD_CCtx`, since `inBuffSize > 0` is
no longer the correct signal for the buffered mode.

lib/compress/zstd_compress_internal.h

@@ -249,6 +249,16 @@ struct ZSTD_CCtx_params_s {
 #define COMPRESS_SEQUENCES_WORKSPACE_SIZE (sizeof(unsigned) * (MaxSeq + 2))
 #define ENTROPY_WORKSPACE_SIZE (HUF_WORKSPACE_SIZE + COMPRESS_SEQUENCES_WORKSPACE_SIZE)
 
+/**
+ * Indicates whether this compression proceeds directly from user-provided
+ * source buffer to user-provided destination buffer (ZSTDb_not_buffered), or
+ * whether the context needs to buffer the input/output (ZSTDb_buffered).
+ */
+typedef enum {
+    ZSTDb_not_buffered,
+    ZSTDb_buffered
+} ZSTD_buffered_policy_e;
+
 struct ZSTD_CCtx_s {
     ZSTD_compressionStage_e stage;
     int cParamsChanged;                  /* == 1 if cParams(except wlog) or compression level are changed in requestedParams. Triggers transmission of new params to ZSTDMT (if available) then reset to 0. */
@@ -278,6 +288,9 @@ struct ZSTD_CCtx_s {
     ZSTD_blockState_t blockState;
     U32* entropyWorkspace;  /* entropy workspace of ENTROPY_WORKSPACE_SIZE bytes */
 
+    /* Wether we are streaming or not */
+    ZSTD_buffered_policy_e bufferedPolicy;
+
     /* streaming */
     char*  inBuff;
     size_t inBuffSize;