new method to deal with offset==0

in this new method, when an `offset==0` is detected,
it's converted into (size_t)(-1), instead of 1.

The logic is that (size_t)(-1) is effectively an extremely large positive number,
which will not pass the offset distance test at next stage (`execSequence()`).
Checked the source code, and offset is always checked (as it should),
using a formula which is not vulnerable to arithmetic overflow:
```
RETURN_ERROR_IF(sequence.offset > (size_t)(oLitEnd - virtualStart),
```

The benefit is that such a case (offset==0) is always detected as corrupted data
as opposed to relying on the checksum to detect the error.

lib/decompress/zstd_decompress_block.c

@@ -1305,7 +1305,7 @@ ZSTD_decodeSequence(seqState_t* seqState, const ZSTD_longOffset_e longOffsets, c
                 } else {
                     offset = ofBase + ll0 + BIT_readBitsFast(&seqState->DStream, 1);
                     {   size_t temp = (offset==3) ? seqState->prevOffset[0] - 1 : seqState->prevOffset[offset];
-                        temp += !temp;   /* 0 is not valid; input is corrupted; force offset to 1 */
+                        temp -= !temp; /* 0 is not valid: input corrupted => force offset to -1 => corruption detected at execSequence */
                         if (offset != 1) seqState->prevOffset[2] = seqState->prevOffset[1];
                         seqState->prevOffset[1] = seqState->prevOffset[0];
                         seqState->prevOffset[0] = offset = temp;