elbert weutil: bound fscanf field width when reading local MAC

[rootkiller6788]

Problem

read_local_mac() in elbert_weutil.c reads the management MAC address with:

fscanf(fp, "%s", buffer);

%s has no field-width limit, so a longer-than-expected token in /sys/class/net/eth0/address would overflow buffer. The only caller passes char local_mac[20], so the buffer holds at most 19 characters plus the NUL terminator.

Fix

Bound the field width to match the buffer:

fscanf(fp, "%19s", buffer);

A real MAC address (17 chars, e.g. 00:11:22:33:44:55) still fits, but the read can no longer overflow the buffer if the sysfs node ever returns a longer token.

Fixes #126.

[meta-codesync]

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