Fix Signature max length check

[igraziatto]

Closes https://github.com/vanilla/support/issues/728

Steps to test:

• Enable Signature addon
• Set the maximum number of characters to 5
• Try and update a signature to 0 or 1 characters
• You will get a "Signature is X characters too long" error
• Checkout this branch try again to see the issue resolve

Further testing:

• Try the same process above in a different posting format

[blutarch]

Tested locally and it works.

[initvector]

Two approvals. I am assuming this is cool to merge.