Avoid overflow in textbox

Count() returns unsigned values.  Since, we subtract 1 from this value,
if the count is 0, we will underflow the unsigned value, creating an
extremely large value that we multiply by the interline spacing.

common/eda_text.cpp

@@ -316,6 +316,8 @@ EDA_RECT EDA_TEXT::GetTextBox( int aLine, bool aInvertY ) const
 
         // interline spacing is only *between* lines, so total height is the height of the first
         // line plus the interline distance (with interline spacing) for all subsequent lines
+        // Don't add interline spacing to empty textboxes
+        if( strings.GetCount() )
             textsize.y += KiROUND( ( strings.GetCount() - 1 ) * interline );
     }