EXT-8226 FIXED Potential fix for a crash at startup in LLIMWellWindow::findIMChiclet().

Bug reason: LLChicletPanel::onCurrentVoiceChannelChanged() was called at startup, which, in turn, called LLIMWellWindow::findIMChiclet().
Apparently, LLIMWellWindow::mMessageList was not initialized yet, so dereferencing the null pointer caused the crash.

I couldn't reproduce the crash so I've just added defensive checks (just for any case)
and moved binding LLIMWellWindow::findIMChiclet() to sFindChicletsSignal from the constructor to postBuild().

Reviewed by Alexei Arabadji at https://codereview.productengine.com/secondlife/r/692/

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branch : product-engine