Fix possibility of getting invalid fixed-pitch advance size.

FT_Get_Advance() returns zero if caching and the ft_size has been culled. The only way I've seen to trigger this bug:

• Have a layout that includes both Properties and a Text Editor with text inside.
• 2D Zoom the Properties editor (Ctrl-Middle Mouse drag).
• Click inside the Text Editor and all the text will bunch up along the right side
• Ctrl-Middle Mouse scroll to change Text Editor font size will fix it.

This just moves a blf_ensure_size() up a few lines to ensure the size is there when this function is called. In a nutshell the zooming caused the sizes to be culled. Then while setting up the glyph cache again to draw the Text Editor the call to FT_Get_Advance() returned zero which is used for the fixed-pitch advance size.

A very simple fix and own code, so will likely commit without review.