fixed a bug in zstreamtest

decoder output buffer would receive a wrong size.

In previous version, ZSTD_decompressStream() would blindly trust the caller that pos <= size.
In this version, this condition is actively checked,
and the function returns an error code if this condition is not respected.

This check could also be done with an assert(),
but since this is a user-facing interface, it seems better to keep this check at runtime.

tests/zstreamtest.c

@@ -914,7 +914,7 @@ static int fuzzerTests(U32 seed, U32 nbTests, unsigned startTest, double compres
                 size_t const randomDstSize = FUZ_randomLength(&lseed, maxSampleLog);
                 size_t const dstBuffSize = MIN(dstBufferSize - totalGenSize, randomDstSize);
                 inBuff.size = inBuff.pos + readCSrcSize;
-                outBuff.size = inBuff.pos + dstBuffSize;
+                outBuff.size = outBuff.pos + dstBuffSize;
                 decompressionResult = ZSTD_decompressStream(zd, &outBuff, &inBuff);
                 if (ZSTD_getErrorCode(decompressionResult) == ZSTD_error_checksum_wrong) {
                     DISPLAY("checksum error : \n");