net: ipv4: fib_semantics: fix uninitialized variable

fix an uninitialized variable:

  CC      net/ipv4/fib_semantics.o
net/ipv4/fib_semantics.c: In function 'fib_check_nh_v4_gw':
net/ipv4/fib_semantics.c:1027:12: warning: 'err' may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (!tbl || err) {
            ^~

Signed-off-by: Enrico Weigelt <info@metux.net>
Signed-off-by: David S. Miller <davem@davemloft.net>