selftests: forwarding: Handle 0 for packet difference in multipath tests

If the packet stats have a difference of 0, the test output shows:
INFO: Expected ratio 2.00 Measured ratio
Runtime error (func=(main), adr=9): Divide by zero
(standard_in) 2: syntax error
(standard_in) 1: syntax error
./router_multipath.sh: line 187: test: : integer expression expected
TEST: Multipath                                                     [FAIL]
	Too large discrepancy between expected and measured ratios

Handle the 0 and display a cleaner message:
INFO: Running IPv6 multipath tests
TEST: Multipath                                                     [FAIL]
	Packet difference is 0
Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/forwarding/router_multipath.sh

@@ -168,6 +168,13 @@ multipath_eval()
 
        RET=0
 
+       if [[ "$packets_rp12" -eq "0" || "$packets_rp13" -eq "0" ]]; then
+              check_err 1 "Packet difference is 0"
+              log_test "Multipath"
+              log_info "Expected ratio $weights_ratio"
+              return
+       fi
+
        if [[ "$weight_rp12" -gt "$weight_rp13" ]]; then
                weights_ratio=$(echo "scale=2; $weight_rp12 / $weight_rp13" \
 		       | bc -l)