Fix attempting to call IsValid from a possibly nil object

[StyledStrike]

• Use IsValid(this) instead of this:IsValid() when calling webaudio:isValid(). Otherwise, say we want to check if we created a webaudio previously and want to delete it before creating a new one, like on this snippet:

@persist WA_TTS:webaudio

...

if (WA_TTS:isValid()) {
    WA_TTS:destroy()
}

WA_TTS = webAudio("...")
WA_TTS:setParent(entity())
WA_TTS:play()

Will throw this error:

sv: Expression 2 (generic): entities/gmod_wire_expression2/core/custom/webaudio.lua:228: attempt to index local 'this' (a nil value)

[Vurv78]

Nice thanks