lpm6: fix compilation with -Og

lpm6: fix compilation with -Og

The compilation with gcc-6.3.0 and EXTRA_CFLAGS=-Og gives the following
error:

  CC rte_lpm6.o
  rte_lpm6.c: In function ‘rte_lpm6_add_v1705’:
  rte_lpm6.c:442:11: error: ‘tbl_next’ may be used uninitialized in
                             this function [-Werror=maybe-uninitialized]
     if (!tbl[tbl_index].valid) {
             ^
  rte_lpm6.c:521:29: note: ‘tbl_next’ was declared here
    struct rte_lpm6_tbl_entry *tbl_next;
                               ^~~~~~~~

This is a false positive from gcc. Fix it by initializing tbl_next
to NULL.

Fixes: 5c510e13a9cb ("lpm: add IPv6 support")
Cc: stable@dpdk.org
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>