Fix misplaced Brotli state NULL pointer check

`BrotliDecoderCreateInstance()` can return NULL (e.g. if it can't
allocate). `BrotliDecoderAttachDictionary()` accesses the state and will
trigger undefined behavior if it is NULL; therefore, we should check for
that *before* calling `BrotliDecoderAttachDictionary()`.

Bug: b:305733200
Change-Id: I1e09df6a1888e974b3682963effb78e9e723e7e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4952174
Reviewed-by: Adam Rice <ricea@chromium.org>
Auto-Submit: Etienne Dechamps <edechamps@google.com>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1211428}