fib: refresh adj pointer after fib_walk_sync due to possible realloc

fib_walk_sync may call adj_alloc which may cause adj_pool to expand. When
that happens, any previous frame which still use the old adj pointer needs to
refresh. Failure to do so may access or update to the old adj memory
unintentionally and crash mysteriously.

Type: fix
Ticket: VPPSUPP-54

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I173dec4c5ce81c6e26c4fe011b894a7345901b24