Fix miri reported UB in FuncRef and ExternRef conversions

[Robbepop]

Closes #1200.

This PR implements the solution brought forward in this comment:
#1200 (comment)

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.81%. Comparing base (c0f79e1) to head (576d843).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/externref.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1201      +/-   ##
==========================================
- Coverage   80.82%   80.81%   -0.01%     
==========================================
  Files         300      299       -1     
  Lines       24906    24902       -4     
==========================================
- Hits        20130    20125       -5     
- Misses       4776     4777       +1     

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[Robbepop]

The Miri (spec) job unfortunately is failing but not because some tests were failing but because it ran out of time which is partially expected and sometimes happens. We need a better solution than a hard-time out in the future.

In conclusion: The miri CI jobs pass again, generally.