Fix NPE with `cpp-restsdk` client generator

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Fix the following NPE:

[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found, returning only the first one
[main] WARN  o.o.codegen.utils.ModelUtils - Multiple schemas found, returning only the first one
[main] INFO  o.o.codegen.DefaultGenerator - Model periodicPaymentInitiationMultipartBody (marked as unused due to form parameters) is generated due to skipFormModel=false (default)
[main] INFO  o.o.codegen.DefaultGenerator - Model periodicPaymentInitiation_xml-Part2-standingorderType_json (marked as unused due to form parameters) is generated due to skipFormModel=false (default)
Exception in thread "main" java.lang.NullPointerException
	at org.openapitools.codegen.languages.CppRestSdkClientCodegen.processParentPropertiesInChildModel(CppRestSdkClientCodegen.java:438)
	at org.openapitools.codegen.languages.CppRestSdkClientCodegen.postProcessParentModels(CppRestSdkClientCodegen.java:425)
	at org.openapitools.codegen.languages.CppRestSdkClientCodegen.postProcessAllModels(CppRestSdkClientCodegen.java:416)
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:439)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:865)
	at org.openapitools.codegen.cmd.Generate.run(Generate.java:349)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:62)

cc @ravinikam (2017/07) @stkrwork (2017/07) @fvarose (2017/11) @etherealjoy (2018/02) @MartinDelille (2018/03)