nextToUpdate3 does not need to be maintained outside of zstd_opt.c

It's re-synchronized with nextToUpdate at beginning of each block.
It only needs to be tracked from within zstd_opt block parser.

Made the logic clear, so that no code tried to maintain this variable.

An even better solution would be to make nextToUpdate3
an internal variable of ZSTD_compressBlock_opt_generic().
That would make it possible to remove it from ZSTD_matchState_t,
thus restricting its visibility to only where it's actually useful.

This would require deeper changes though,
since the matchState is the natural structure to transport parameters into and inside the parser.

lib/compress/zstd_compress_internal.h

@@ -134,9 +134,9 @@ typedef struct {
 typedef struct ZSTD_matchState_t ZSTD_matchState_t;
 struct ZSTD_matchState_t {
     ZSTD_window_t window;   /* State for window round buffer management */
-    U32 loadedDictEnd;      /* index of end of dictionary, within context's referential. When dict referential is copied into active context (i.e. not attached), effectively same value as dictSize, since referential starts at zero */
+    U32 loadedDictEnd;      /* index of end of dictionary, within context's referential. When dict referential is copied into active context (i.e. not attached), effectively same value as dictSize, since referential starts from zero */
     U32 nextToUpdate;       /* index from which to continue table update */
-    U32 nextToUpdate3;      /* index from which to continue table update of hashTable3 */
+    U32 nextToUpdate3;      /* index from which to continue table update of hashTable3. synchronized with nextToUpdate at block's start, so no need to maintain outside of parser. Used by opt parser only */
     U32 hashLog3;           /* dispatch table : larger == faster, more memory */
     U32* hashTable;
     U32* hashTable3;