Fix #181 #183 memory corruption when unpacking unknown enum or repacking 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 or
• repacks the same enum (refcount too low)

...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.

Fixes #181 and probably #183 as well.

With regards to the repacking issue, we need to add to the ref count for some reason when unpacking an enum value. That is why I replaced ZVAL_OBJ with ZVAL_OBJ_COPY since the difference is exactly that:

#define ZVAL_OBJ(z, o) do {						\
		zval *__z = (z);						\
		Z_OBJ_P(__z) = (o);						\
		Z_TYPE_INFO_P(__z) = IS_OBJECT_EX;		\
	} while (0)

#define ZVAL_OBJ_COPY(z, o) do {				\
		zval *__z = (z);						\
		zend_object *__o = (o);					\
		GC_ADDREF(__o);							\
		Z_OBJ_P(__z) = __o;						\
		Z_TYPE_INFO_P(__z) = IS_OBJECT_EX;		\
	} while (0)

Excerpt from php/php-src/Zend/zend_types.h#L1138

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.51%. Comparing base (712cc13) to head (35d4381).
⚠️ Report is 86 commits behind head on master.

Files with missing lines	Patch %	Lines
msgpack_pack.c	0.00%	1 Missing ⚠️
msgpack_unpack.c	88.88%	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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