Fix variable in systemd not initialized issue

[daxia16]

Why I did it

The variable is not initialized in src/systemd-sonic-generator/systemd-sonic-generator.c. I fixed this one.

How I did it

Assure pointer is initialized to NULL when defined and before use.

How to verify it

Which release branch to backport (provide reason below if selected)

• [ ] 201811
• [ ] 201911
• [ ] 202006
• [ ] 202012
• [ ] 202106
• [ ] 202111
• [ ] 202205

Description for the changelog

Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: daxia16 (56ceb4c3bc7b2dcebd830d455be6842f847e37eb)

[liat-grozovik]

@daxia16 please check EasyCLA

[mssonicbld]

@daxia16 PR conflicts with 202211 branch