fix decodecorpus incorrect frame generation

fix #1379
decodecorpus was generating one extraneous byte when `nbSeq==0`.
This is disallowed by the specification.

The reference decoder was just skipping the extraneous byte.
It is now stricter, and flag such situation as an error.

tests/decodecorpus.c

@@ -823,15 +823,14 @@ static size_t writeSequences(U32* seed, frame_t* frame, seqStore_t* seqStorePtr,
     else if (nbSeq < LONGNBSEQ) op[0] = (BYTE)((nbSeq>>8) + 0x80), op[1] = (BYTE)nbSeq, op+=2;
     else op[0]=0xFF, MEM_writeLE16(op+1, (U16)(nbSeq - LONGNBSEQ)), op+=3;
 
-    /* seqHead : flags for FSE encoding type */
-    seqHead = op++;
-
     if (nbSeq==0) {
         frame->data = op;
-
         return 0;
     }
 
+    /* seqHead : flags for FSE encoding type */
+    seqHead = op++;
+
     /* convert length/distances into codes */
     ZSTD_seqToCodes(seqStorePtr);