Add CodeQL workflow for GitHub code scanning

.github/workflows/codeql.yml

@@ -0,0 +1,45 @@
+name: "CodeQL"
+
+on:
+  push:
+    branches: [ "master" ]
+  pull_request:
+    branches: [ "master" ]
+  schedule:
+    - cron: "5 19 * * 2"
+
+jobs:
+  analyze:
+    name: Analyze
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [ cpp ]
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Configure (cpp)
+        if: ${{ matrix.language == 'cpp' }}
+        run: cmake -S $GITHUB_WORKSPACE -B $GITHUB_WORKSPACE/_lgtm_build_dir -DINTX_TESTING=1
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v2
+        with:
+          languages: ${{ matrix.language }}
+          queries: +security-and-quality
+
+      - name: Autobuild
+        uses: github/codeql-action/autobuild@v2
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@v2
+        with:
+          category: "/language:${{ matrix.language }}"

test/unittests/test_uint256.cpp

@@ -15,6 +15,7 @@ constexpr uint64_t minimal[] = {
     0x7fffffffffffffff,
     0x8000000000000000,
     0xaaaaaaaaaaaaaaaa,
+    0xfffffffffffffffd,
     0xfffffffffffffffe,
     0xffffffffffffffff,
 };