[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

tests/fuzz/stream_round_trip.c

@@ -96,6 +96,13 @@ static size_t compress(uint8_t *dst, size_t capacity,
                     }
                     break;
                 }
+                case 4: {
+                    ZSTD_inBuffer nullIn = { NULL, 0, 0 };
+                    ZSTD_outBuffer nullOut = { NULL, 0, 0 };
+                    size_t const ret = ZSTD_compressStream2(cctx, &nullOut, &nullIn, ZSTD_e_continue);
+                    FUZZ_ZASSERT(ret);
+                }
+                /* fall-through */
                 default: {
                     size_t const ret =
                         ZSTD_compressStream2(cctx, &out, &in, ZSTD_e_continue);