Fix potential UB

[Robbepop]

• As reported by miri when testing for --doc tests.
• We cannot use NonNull there since NonNull is missing the respective offset method, yet, as discussed and requested here: NonNull methods for pointer offset, add, sub rust-lang/rust#72429
• This PR also adds a miri --doc CI test run since that is how we discovered this potential UB.

[paritytech-cicd-pr]

BENCHMARKS

	NATIVE			WASMTIME
BENCHMARK	MASTER	PR	DIFF	MASTER	PR	DIFF	WASMTIME OVERHEAD
execute/ bare_call_0	1.28ms	1.29ms	⚪ 0.79%	1.15ms	1.06ms	🔴 -8.02%	🟢 -17%
execute/ bare_call_0/typed	704.45µs	698.55µs	⚪ -0.73%	536.03µs	561.95µs	🔴 4.80%	🟢 -20%
execute/ bare_call_1	1.45ms	1.40ms	🔴 -3.07%	1.36ms	1.33ms	🔴 -2.12%	🟢 -5%
execute/ bare_call_16	2.86ms	2.94ms	🔴 2.82%	5.15ms	4.83ms	🔴 -5.30%	🟡 65%
execute/ bare_call_16/typed	1.29ms	1.30ms	⚪ 0.78%	2.49ms	2.52ms	🔴 1.44%	🟡 94%
execute/ bare_call_1/typed	733.81µs	722.60µs	🟢 -1.56%	694.19µs	730.18µs	🔴 5.18%	🟢 1%
execute/ bare_call_4	1.82ms	1.75ms	⚪ -1.08%	2.16ms	2.09ms	🔴 -3.40%	🟢 19%
execute/ bare_call_4/typed	894.73µs	890.69µs	⚪ -0.50%	995.54µs	1.04ms	🔴 4.30%	🟢 17%
execute/ count_until	931.07µs	887.63µs	🟢 -4.59%	2.38ms	2.27ms	🟢 -4.48%	🔴 156%
execute/ factorial_iterative	393.64ns	393.00ns	⚪ -0.08%	988.67ns	950.95ns	🟢 -3.90%	🔴 142%
execute/ factorial_recursive	748.41ns	784.81ns	🔴 4.85%	1.60µs	1.60µs	⚪ 0.34%	🔴 104%
execute/ fib_iterative	1.79ms	1.80ms	⚪ 0.29%	5.11ms	5.02ms	🟢 -1.68%	🔴 179%
execute/ fib_recursive	6.84ms	7.05ms	🔴 4.20%	14.24ms	14.47ms	🔴 1.65%	🔴 105%
execute/ global_bump	1.45ms	1.44ms	⚪ -0.95%	3.68ms	3.65ms	⚪ -0.93%	🔴 153%
execute/ host_calls	32.46µs	32.35µs	⚪ -0.07%	45.26µs	44.91µs	⚪ -0.98%	🟢 39%
execute/ memory_fill	1.53ms	1.54ms	⚪ 0.20%	4.33ms	4.14ms	🟢 -4.42%	🔴 169%
execute/ memory_sum	1.52ms	1.52ms	⚪ -0.51%	4.36ms	4.15ms	🟢 -4.76%	🔴 174%
execute/ memory_vec_add	3.02ms	3.04ms	⚪ 0.31%	9.00ms	8.76ms	🟢 -2.68%	🔴 188%
execute/ recursive_is_even	1.32ms	1.32ms	⚪ -0.65%	2.58ms	2.60ms	⚪ 0.50%	🟡 97%
execute/ recursive_ok	174.51µs	178.63µs	🔴 2.37%	358.79µs	359.74µs	⚪ 0.22%	🔴 101%
execute/ recursive_scan	217.44µs	221.14µs	⚪ 1.15%	462.69µs	464.86µs	⚪ 0.54%	🔴 110%
execute/ recursive_trap	17.22µs	18.42µs	🔴 6.98%	35.76µs	35.62µs	⚪ -0.43%	🟡 93%
execute/ regex_redux	651.29µs	697.38µs	🔴 6.90%	1.60ms	1.62ms	⚪ 1.18%	🔴 132%
execute/ rev_complement	586.24µs	600.11µs	🔴 2.30%	1.56ms	1.53ms	🟢 -1.35%	🔴 156%
execute/ tiny_keccak	456.66µs	452.82µs	⚪ -0.51%	1.45ms	1.41ms	🟢 -3.02%	🔴 211%
execute/ trunc_f2i	1.07ms	1.07ms	⚪ -0.19%	2.73ms	2.63ms	🟢 -3.58%	🔴 146%
instantiate/ wasm_kernel	66.21µs	67.62µs	⚪ 1.54%	127.35µs	126.39µs	⚪ -0.95%	🟡 87%
translate/ wasm_kernel	4.83ms	4.81ms	⚪ -0.40%	10.22ms	9.82ms	🟢 -3.97%	🔴 104%

Link to pipeline

[codecov-commenter]

Codecov Report

Merging #524 (c3c5017) into master (dab2565) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage   79.38%   79.38%   -0.01%     
==========================================
  Files          74       74              
  Lines        6278     6276       -2     
==========================================
- Hits         4984     4982       -2     
  Misses       1294     1294              

Impacted Files	Coverage Δ
crates/wasmi/src/engine/code_map.rs	81.57% <100.00%> (-0.93%)	⬇️

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