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>