slurp_battery_info: Fix reading uninitialised memory (#531)

Fixes valgrind-found bug of the `for (walk = buf, ...` loop
reading all of `buf` even though `buf` is null-terminated string
(an only partly initialised char array).

    valgrind ./i3status -c ../etc/i3status.conf --run-once

    Conditional jump or move depends on uninitialised value(s)
      at 0x40F15A: slurp_battery_info (print_battery_info.c:164)
      by 0x40FA07: slurp_all_batteries (print_battery_info.c:558)
      by 0x40FCA6: print_battery_info (print_battery_info.c:612)
      by 0x409CA2: main (i3status.c:753)

src/general.c

@@ -19,7 +19,8 @@
     }
 
 /*
- * Reads size bytes into the destination buffer from filename.
+ * Reads (size - 1) bytes into the destination buffer from filename,
+ * and null-terminate it.
  *
  * On success, true is returned. Otherwise, false is returned and the content
  * of destination is left untouched.

src/print_battery_info.c

@@ -161,6 +161,11 @@ static bool slurp_battery_info(battery_info_ctx_t *ctx, struct battery_info *bat
     }
 
     for (walk = buf, last = buf; (walk - buf) < 1024; walk++) {
+        // `*walk` (slice of `buf`) is only initialised until `null` written by `slurp()`
+        if (*walk == '\0') {
+            break;
+        }
+
         if (*walk == '\n') {
             last = walk + 1;
             continue;