Fix battery indicator on systems without POWER_SUPPLY_VOLTAGE_NOW.

Long description:

In my case, the voltage variable would stay initialized as -1,
which caused the calculation of battery charge percentage to be
incorrect (I would get the message that there is no battery present
or even -0% charge).

I have no idea how this would affect other systems, since I don't
have a chance to test this.

To apply this patch, use:
curl http://cr.i3wm.org/patch/638/raw.patch | git am

@@ -124,8 +124,11 @@ void print_battery_info(yajl_gen json_gen, char *buffer, int number, const char

          * ampere to watt */

         if (!watt_as_unit) {

             present_rate = (((float)voltage / 1000.0) * ((float)present_rate / 1000.0));

-            remaining = (((float)voltage / 1000.0) * ((float)remaining / 1000.0));

-            full_design = (((float)voltage / 1000.0) * ((float)full_design / 1000.0));

+

+            if (voltage != -1) {

+                remaining = (((float)voltage / 1000.0) * ((float)remaining / 1000.0));

+                full_design = (((float)voltage / 1000.0) * ((float)full_design / 1000.0));

+            }

         }

         if ((full_design == -1) || (remaining == -1)) {