pass sizeof(struct sockaddr_in) for AF_INET instead of always using
sizeof(struct sockaddr_in6). also skip getnameinfo entirely for
non-matching interfaces rather than calling it for every entry.

replace strcpy of interface name into ifr_name (IFNAMSIZ=16) with
bounds-checked snprintf. add one byte to resp buffer so NUL-terminating
the SSID at resp boundary does not write out of bounds.

if no "vol" device exists in the mixer, v was used uninitialized.
return early on match and return NULL with a warning otherwise.

the swap entry loop read the first entry repeatedly instead of
iterating with sep[i]. also free the allocation on error paths
and fix "SWAP_STATA" typo in the warning message.

"Not charging" is 12 characters; %12[a-zA-Z ] reads up to 12 chars
plus a NUL terminator (13 bytes) into a 12-byte buffer. Increase
state buffer to 13 in both battery_state and battery_remaining.

ifindex() caches the result of ioctl(SIOCGIFINDEX) and never
retries it, since the interface name strcmp succeeds after the
first call. if the interface does not exist when slstatus starts
(e.g. iwd with default config destroys/recreates interfaces
depending on service state), the ioctl fails and the stale
result is returned on all subsequent calls.

based on a patch by ryhpez, with input from Taha Medhous