oeqa ping.py: fail test if target IP address has not been set

It is possible to call exported tests with --target-ip set to ":22"
where IP address is not set at all. Detect this case and fail the test
instead of calling ping without an IP address.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>