Berry fix walrus bug when assigning to self

[s-hadinger]

Description:

Fix Berry walrus regression when assigning to self (found by sfromis):

class confused_walrus
  var b
  def f()
    var c = 1
    if self.b := true
      c = 2
    end
    return self
  end
end
print(confused_walrus().f())
# prints 'true' instead of the instance

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.6
• The code change is tested and works with Tasmota core ESP32 V.2.0.14
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass