AutomatableModel: remove bogus assigment if step size is 0

No idea why we set value to m_minValue in case the step size is 0. There
shouldn't be models with step size 0 nevertheless.
AutomatableModel: use nearbyintf() for rounding

For some reason roundf() causes an exception on Win32 when calling with
negative values. This caused LMMS to crash when turning an external MIDI
pitch wheel downwards.

Closes #281.