Don't Attach Very Small Dictionaries

Don't Attach Very Small Dictionaries

Fixes a mismatch in behavior between loading into the context (via
`LZ4_loadDict()`) a very small (<= 4 bytes) non-contiguous dictionary, versus
attaching it with `LZ4_attach_dictionary()`.

Before this patch, this divergence could be reproduced by running

```
make -C tests fuzzer MOREFLAGS="-m32"
tests/fuzzer -v -s1239 -t3146
```

Making sure these two paths behave exactly identically is an easy way to test
the correctness of the attach path, so it's desirable that this remain an
unpolluted, high signal test.