[fuzz] Allow zero sized buffers for streaming fuzzers (#1945)

* Allow zero sized buffers in `stream_decompress`. Ensure that we never have two
  zero sized buffers in a row so we guarantee forwards progress.
* Make case 4 in `stream_round_trip` do a zero sized buffers call followed by
  a full call to guarantee forwards progress.
* Fix `limitCopy()` in legacy decoders.
* Fix memcpy in `zstdmt_compress.c`.

Catches the bug fixed in PR #1939

lib/legacy/zstd_v06.c

@@ -4000,7 +4000,9 @@ size_t ZBUFFv06_decompressInit(ZBUFFv06_DCtx* zbd)
 MEM_STATIC size_t ZBUFFv06_limitCopy(void* dst, size_t dstCapacity, const void* src, size_t srcSize)
 {
     size_t length = MIN(dstCapacity, srcSize);
-    memcpy(dst, src, length);
+    if (length > 0) {
+        memcpy(dst, src, length);
+    }
     return length;
 }