ext/snmp: fix infinite loop in snprint_value retry when val_len is zero

[thomasvincent]

When an SNMP variable has val_len == 0 (valid for empty octet strings),
the snprint_value retry loop doubles val_len on each iteration
(val_len *= 2). Since 0 * 2 == 0, val_len never grows, the
allocated buffer stays at 1 byte, and the 512k break condition is
never reached. This causes an infinite loop consuming CPU.

Fix by clamping val_len to at least sizeof(sbuf) before the loop,
ensuring the doubling produces meaningful growth past the initial stack
buffer size.

[thomasvincent]

Closing to re-evaluate the appropriate disclosure channel.