Upon garbage collection, nested struct property is corrupted

[hunterlester]

Minimal reproduction of the issue: https://github.com/hunterlester/minimum-ref-struct-corruption

Notice that the 3rd line of the log output is not corrupted whereas the 2nd nested value is corrupted:

Running garbage collection...
authGranted object afte gc:  { name: '�_n9a\u0002', 'ref.buffer': <Buffer@0x00000261396F3910 18 86 6c 39 61 02 00 00> }
Unnested access container entry after gc:  { name: 'apps/net.maidsafe.examples.mailtutorial', 'ref.buffer': <Buffer@0x00000261396F3B10 60 68 6e 39 61 02 00 00> }
Globally assigned values after gc:  apps/net.maidsafe.examples.mailtutorial  _publicNames

[ZFail]

Working online reproduction that shows the problem: https://repl.it/repls/FarflungTallInterfacestandard
Workaround: revert optimization changes from this commit: 252ad69#diff-7e303bd4db5b70ed2ea32bff040ca830
Or my pr: #37