Fix memory corruption when unpacking unknown enum

[cracksalad]

If one unpacks a serialized enum value and...

• the corresponding enum definition could not be loaded (not required nor autoloadable) or
• the type name (say test\MyType) is not an enum but a normal class

...there is no warning or error thrown but one would experience memory corruption instead. I am talking about variables having values which are not allowed by their types and something like that - really scary.

Definitely related to #183 and maybe also to #181

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.51%. Comparing base (712cc13) to head (5bded50).
⚠️ Report is 86 commits behind head on master.

Files with missing lines	Patch %	Lines
msgpack_unpack.c	87.50%	0 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   73.88%   68.51%   -5.37%     
==========================================
  Files           6        7       +1     
  Lines        1191     1350     +159     
  Branches        0      247     +247     
==========================================
+ Hits          880      925      +45     
+ Misses        311      310       -1     
- Partials        0      115     +115     

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