bgpd: fix memory leak when reconfiguring a route distinguisher (backport #17669)

[mergify]

A memory leak happens when reconfiguring an already configured route distinguisher on an L3VPN BGP instance. Fix this by freeing the previous route distinguisher.

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This is an automatic backport of pull request #17669 done by Mergify.

[mergify]

Cherry-pick of 0dd9628 has failed:

On branch mergify/bp/stable/8.5/pr-17669
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	both modified:   bgpd/bgp_vty.c

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