GUI: Fix compilation with clang and C++11.

newDictLine is allocated with calloc in line 856 thus there is no need to
initialize any entry.
GUI: Clean up calloc use in PredictiveDialog.

Instead of manually multiplying the entry count with the entry size we simply
use both parameters of calloc as intended now.
GUI: Initialize PredictiveDialog::_predictiveResult.

This fixes garbage output when canceling the predictive dialog in AGI when
nothing was entered.
GUI: Do not return current input on cancel in PredictiveDialog.

Returning the currently displayed input when you click cancel is confusing
behavior in my eyes.