Reset posInSequence when using ZSTD_referenceExternalSequences()

lib/compress/zstd_compress.c

@@ -2853,6 +2853,7 @@ size_t ZSTD_referenceExternalSequences(ZSTD_CCtx* cctx, rawSeq* seq, size_t nbSe
     cctx->externSeqStore.size = nbSeq;
     cctx->externSeqStore.capacity = nbSeq;
     cctx->externSeqStore.pos = 0;
+    cctx->externSeqStore.posInSequence = 0;
     return 0;
 }