[Mellanox] [platform API] Fix “local variable 'label_port' referenced before assignment” error

[keboliu]

- Why I did it

In rare case can see that xcvrd failed due to "UnboundLocalError: local variable 'label_port' referenced before assignment"

- How I did it

Init "label_port" as None at the beginning of the function, to avoid the case that "label_port" not assigned.

- How to verify it

run SFP related test on Mellanox platforms.

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

• 201811
• 201911
• 202006
• 202012

- Description for the changelog

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

[keboliu]

retest vsimage please

[jleveque]

@keboliu: I assume we should also cherry-pick this into the 202012 branch, no?

[keboliu]

@keboliu: I assume we should also cherry-pick this into the 202012 branch, no?

@jleveque, although this happened in a very rare case, it shall be included in 20191.

[jleveque]

@jleveque, although this happened in a very rare case, it shall be included in 20191.

But what about 202012? I assume the rare issue exists there too, no (we just created this branch at the end of December)?

[keboliu]

@jleveque, although this happened in a very rare case, it shall be included in 20191.

But what about 202012? I assume the rare issue exists there too, no (we just created this branch at the end of December)?

202012 also need to be included, I thought 202012 will be automatically synced with the master for a period

[jleveque]

@jleveque, although this happened in a very rare case, it shall be included in 20191.

But what about 202012? I assume the rare issue exists there too, no (we just created this branch at the end of December)?

202012 also need to be included, I thought 202012 will be automatically synced with the master for a period

Just to be certain that the PR gets included in the branch, please add the "Request for 202012 branch" to any relevant PRs so that we can track.