certain fonts with less pixel size does not have linearadvance value and so it is over written

I am using certain unicode fonts and when the pixel size is really less eg. 9,
In function QFontEngineFT::recalcAdvances under qfontengine_ft.cpp, the g->linearadvance value is always 0.So there is no spacing between characters and it is overwrites.But for the same font file, if the font size is 20 it seems to work fine.There is a valid linear advance value for higher pixel size.I am using windows batang file.I have attached web.ttf font file due to size limitations.