osc.lua: fix calculation for slider's min-max average

Average of two numbers is given by the sum of the two numbers
divided by two. It's gone unnoticed because `s_min` is
basically always zero.

player/lua/osc.lua

@@ -818,7 +818,7 @@ function render_elements(master_ass)
                             elseif (sliderpos > (s_max - 3)) then
                                 an = an + 1
                             end
-                        elseif (sliderpos > (s_max-s_min)/2) then
+                        elseif (sliderpos > (s_max+s_min)/2) then
                             an = an + 1
                             tx = tx - 5
                         else