Fix parsing of MAC address in retract string.

Issue:
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The retract string bgp for EVPN routes is of format,
<vn-id>-<mac>,<ip>

The current code for marsing mac-address only strips vn-id and tries to
decode MAC address from string <mac>,<ip>. The decode fails due to presence
of IP address.

Fix:
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Strip the IP address and additional "," using strtok_r and parse the MAC
address