Previously, odd things could happen if a widget was getting fitted into a
negative width or, even worse, width being NaN (not a number)!
This can e.g. happen due to a margin layout which doesn't get enough space to
even draw the margin that it is supposed to add.
Fix this by enforcing a minimum value of 0 for the width and height that a
widget gets fitted into.
Signed-off-by: Uli Schlachter <psychon@znc.in>
db00145406