Fix a problem that points to a folder that doesn't exist

Created by: ackintosh

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\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

When working directory contains . , we can see following error:

$ pwd
/Users/akihito.nakano/src/github.com/ackintosh/openapi-generator-1/
$ ./bin/java-petstore-feign.sh
[main] INFO  o.o.codegen.AbstractGenerator - writing file /Users/akihito/nakano/src/github/com/ackintosh/openapi-generator-1/samples/client/petstore/java/feign/src/main/java/org/openapitools/client/model/AdditionalPropertiesClass.java
Exception in thread "main" java.lang.RuntimeException: Could not generate model 'AdditionalPropertiesClass'
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:509)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:904)
        at org.openapitools.codegen.cmd.Generate.run(Generate.java:354)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)
Caused by: java.io.FileNotFoundException: /Users/akihito/nakano/src/github/com/ackintosh/openapi-generator-1/samples/client/petstore/java/feign/src/main/java/org/openapitools/client/model/AdditionalPropertiesClass.java (No such file or directory)
        at java.io.FileOutputStream.open0(Native Method)
        at java.io.FileOutputStream.open(FileOutputStream.java:270)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:213)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:162)
        at org.openapitools.codegen.AbstractGenerator.writeToFile(AbstractGenerator.java:49)
        at org.openapitools.codegen.DefaultGenerator.processTemplateToFile(DefaultGenerator.java:946)
        at org.openapitools.codegen.DefaultGenerator.generateModel(DefaultGenerator.java:322)
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:498)
        ... 3 more
mkdir: samples/client/petstore/java/feign/src/test/java/org/openapitools/client: File exists

Caused by: java.io.FileNotFoundException: /Users/akihito/nakano/src/github/com/ackintosh/openapi-generator-1/samples/client/petstore/java/feign/src/main/java/org/openapitools/client/model/AdditionalPropertiesClass.java (No such file or directory)

The output points to invalid folder, the dots in the folder path has replaced with slash. 🙄