battery_box.lua: Make "srcstack" local (#525)

fgrosswig

2020-01-06 e90b28895cc28daf1a2e4fb772bd191400cf9e62

battery_box.lua: Make "srcstack" local (#525)

Fixes warning:
Assignment to undeclared global "srcstack" inside a function at battery_box.lua:454

 technic/machines/register/battery_box.lua

@@ -452,7 +452,7 @@
    if inv:is_empty("dst") then
        return batt_charge, false
    end
    srcstack = inv:get_stack("dst", 1)
    local srcstack = inv:get_stack("dst", 1)
    local toolname = srcstack:get_name()
    if technic.power_tools[toolname] == nil then
        return batt_charge, false