Memory leak in MXPredCreate()

[josephevans]

Description

There is a memory leak in MXPredCreate() identified by Valgrind. Details below:

==23612== Conditional jump or move depends on uninitialised value(s)
==23612==    at 0x403D6F5: bcmp (vg_replace_strmem.c:1129)
==23612==    by 0x5E2B759: dmlc::parameter::FieldEntryBase<dmlc::parameter::FieldEntry<float>, float>::Same(void*, std::string const&) const (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x6743CCF: void mxnet::op::ParamParser<mxnet::op::DropoutParam>(nnvm::NodeAttrs*) (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x6169A63: mxnet::UpgradeJSON_Parse(nnvm::Graph) (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x6164545: std::_Function_handler<nnvm::Graph (nnvm::Graph), nnvm::Graph (*)(nnvm::Graph)>::_M_invoke(std::_Any_data const&, nnvm::Graph) (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x616843E: mxnet::LoadLegacyJSONPass(nnvm::Graph) (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x6164545: std::_Function_handler<nnvm::Graph (nnvm::Graph), nnvm::Graph (*)(nnvm::Graph)>::_M_invoke(std::_Any_data const&, nnvm::Graph) (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0xAD5D668: nnvm::ApplyPasses(nnvm::Graph, std::vector<std::string, std::allocator<std::string> > const&) (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x5E4450D: nnvm::ApplyPass(nnvm::Graph, std::string const&) (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x5E53637: _CreatePartialOut(char const*, void const*, int, int, int, unsigned int, char const**, unsigned int const*, unsigned int const*, unsigned int, char const**, int, bool, unsigned int, char const**, int const*, void**) (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x5E564F0: MXPredCreate (in /local/home/pazurkie/mx-live/pyenv/lib/python3.7/site-packages/mxnet/libmxnet.so)
==23612==    by 0x40149B: createPredictor() (foo.cpp:56)