Don't Attach Empty Dict Contents

In weird corner cases, they produce unexpected results...

lib/compress/zstd_compress.c

@@ -1227,6 +1227,10 @@ static size_t ZSTD_resetCCtx_usingCDict(ZSTD_CCtx* cctx,
     }
 
     if (attachDict) {
+        if (cdict->matchState.window.nextSrc - cdict->matchState.window.base == 0) {
+            /* don't even attach dictionaries with no contents */
+            DEBUGLOG(4, "skipping attaching empty dictionary");
+        } else {
             DEBUGLOG(4, "attaching dictionary into context");
             cctx->blockState.matchState.dictMatchState = &cdict->matchState;
 
@@ -1241,6 +1245,7 @@ static size_t ZSTD_resetCCtx_usingCDict(ZSTD_CCtx* cctx,
                 ZSTD_window_clear(&cctx->blockState.matchState.window);
             }
             cctx->blockState.matchState.loadedDictEnd = cctx->blockState.matchState.window.dictLimit;
+        }
     } else {
         DEBUGLOG(4, "copying dictionary into context");
         /* copy tables */