Added miri test action

`miri` actually caught a non-bug (in my opinion), but sure thing, I've
changes the code so that `miri` won't complain.

.github/workflows/miri.yml

@@ -0,0 +1,25 @@
+name: UB test
+
+on:
+  push:
+    branches: [ master ]
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  run_miri:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Setup toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: nightly
+          components: miri
+
+      - name: run miri
+        run: cargo +nightly miri test --many-seeds=0..8