[lib] Allow compression dictionaries with missing symbols

Allow compression to use dictionaries with missing symbols in their
entropy tables. We set the FSE repeat mode to check when there are
missing symbols, and set the FSE repeat mode to valid when all symbols
are present.

Note that when not all symbols are present, the heuristics which favor
dictionary tables for lower compression levels won't activate.

Tested by manually creating a dictionary with missing symbols of every
type, and validing that the compressor rejects it before this change,
and accepts it after this change. Also, I ran the `dictionary_loader`
fuzzer for >1 hour of CPU time without running into cases where
compression succeeds, but decompression fails.

Fixes #2174.

lib/compress/zstd_compress_internal.h

@@ -1045,7 +1045,6 @@ MEM_STATIC void ZSTD_debugTable(const U32* table, U32 max)
  * assumptions : magic number supposed already checked
  *               and dictSize >= 8 */
 size_t ZSTD_loadCEntropy(ZSTD_compressedBlockState_t* bs, void* workspace,
-                         short* offcodeNCount, unsigned* offcodeMaxValue,
                          const void* const dict, size_t dictSize);
 
 void ZSTD_reset_compressedBlockState(ZSTD_compressedBlockState_t* bs);