bgpd: Fix memory leak when creating BMP connection with a source interface

Testing done with:

```
for x in $(seq 1 100000); do vtysh -c 'conf' -c 'router bgp' -c 'bmp targets test' -c 'bmp connect localhost port 123 min-retry 100 max-retry 100 source-interface lo'; done
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit 7d19cb5)

Author: ton31337

bgpd/bgp_bmp.c

@@ -2154,8 +2154,11 @@ DEFPY(bmp_connect,
 	}
 
 	ba = bmp_active_get(bt, hostname, port);
-	if (srcif)
+	if (srcif) {
+		if (ba->ifsrc)
+			XFREE(MTYPE_TMP, ba->ifsrc);
 		ba->ifsrc = XSTRDUP(MTYPE_TMP, srcif);
+	}
 	if (min_retry_str)
 		ba->minretry = min_retry;
 	if (max_retry_str)