Valgrind: fix "Invalid read" - test-kvp-value

==7520== Invalid read of size 8
==7520==    at 0x4E36DCC: guid_copy (guid.cpp:123)
==7520==    by 0x1E3E62: KvpValueImpl::duplicate(KvpValueImpl const&) (kvp-value.cpp:378)
==7520==    by 0x1E31CC: KvpValueImpl::KvpValueImpl(KvpValueImpl const&) (kvp-value.cpp:36)
==7520==    by 0x1FF6C3: KvpValueTest_Copy_Test::TestBody() (test-kvp-value.cpp:74)
...
==7520==  Address 0x7e10f48 is 8 bytes inside a block of size 16 free'd
==7520==    at 0x4846AFF: operator delete(void*, unsigned long) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==7520==    by 0x4E36D98: guid_free (guid.cpp:115)
==7520==    by 0x1E3D04: void delete_visitor::operator()<_gncGuid*>(_gncGuid*&) (kvp-value.cpp:358)
...
==7520==  Block was alloc'd at
==7520==    at 0x4843FA3: operator new(unsigned long) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==7520==    by 0x4E36D5C: guid_malloc (guid.cpp:105)
==7520==    by 0x4E36DC3: guid_copy (guid.cpp:122)
==7520==    by 0x4E36EB0: guid_new (guid.cpp:153)
==7520==    by 0x1FF4DB: KvpValueTest_Copy_Test::TestBody() (test-kvp-value.cpp:68)

libgnucash/engine/test/test-kvp-value.cpp

@@ -70,6 +70,7 @@ TEST (KvpValueTest, Copy)
     v2 = std::unique_ptr<KvpValueImpl> {new KvpValueImpl {*v1}};
     EXPECT_EQ (compare (*v1, *v2), 0);
 
+    guid = guid_new ();
     v1 = std::unique_ptr<KvpValueImpl> {new KvpValueImpl {guid}};
     v2 = std::unique_ptr<KvpValueImpl> {new KvpValueImpl {*v1}};
     /*This should delete the guid*/