Fix: Prevent Null Pointer Dereference and memory leaks in newBrushFamilyFromProtoInternal

[DarnExploit141]

This PR fixes a potential Null Pointer Dereference and memory leak in brush_serialization_jni.cc.

Changes:

• Added a null check for new_id_jstring to prevent crashes if onDecodeTexture returns null.
• Added a null check for GetStringUTFChars to prevent crashes on out-of-memory conditions.
• Added ReleaseStringUTFChars to properly release the JNI memory and prevent memory leaks.

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[DarnExploit141]

While reviewing the fix, I noticed an additional potential null dereference that wasn't covered in this PR:

jstring encoded_id_jstring =
    env->NewStringUTF(std::string(encoded_id).c_str());

NewStringUTF can return null on out-of-memory conditions. If this happens, passing it to CallObjectMethod would result in undefined behavior. A null check here would make the fix more complete:

if (encoded_id_jstring == nullptr) {
    return absl::InternalError("NewStringUTF failed (OOM).");
}

I can include this in a follow-up commit if the maintainers agree.