Crash in TMap struct deserializer

When deserializing a TMap<FString, FString> with the JsonStructDeserializer, the Engine crashes.

1) Deserializer a UStruct containing a TMap<FString, FString> property using FJsonStructDeserializer

UE4Editor_Core!rml::internal::Block::findObjectSize() [d:\depot\ue4\engine\source\thirdparty\inteltbb\inteltbb-4.4u3\src\tbbmalloc\frontend.cpp:2107]
UE4Editor_Core!FMallocTBB::Realloc() [d:\malibu\dev\engine\source\runtime\core\private\hal\malloctbb.cpp:70]
UE4Editor_Serialization!TInlineAllocator<4,FDefaultAllocator>::ForElementType<unsigned int>::ResizeAllocation() [d:\malibu\dev\engine\source\runtime\core\public\containers\containerallocationpolicies.h:636]
UE4Editor_Serialization!FScriptBitArray::ReallocGrow() [d:\malibu\dev\engine\source\runtime\core\public\containers\bitarray.h:1075]
UE4Editor_Serialization!FScriptSparseArray::AddUninitialized() [d:\malibu\dev\engine\source\runtime\core\public\containers\sparsearray.h:953]
UE4Editor_Serialization!FStructDeserializer::Deserialize() [d:\malibu\dev\engine\source\runtime\serialization\private\structdeserializer.cpp:203]
UE4Editor_HttpMessaging!FStructDeserializer::Deserialize() [d:\malibu\dev\engine\source\runtime\serialization\public\structdeserializer.h:81]