Skip addr memcmp if getifaddrs returns a null ifa_addr pointer

[RustedBones]

According to the getifaddrs man page, the ifa_addr contained in the ifaddrs struct can be a null pointer.

When running the org.scalanative.testsuite.javalib.net.NetworkInterfaceTest unit-test on my machine, I have the following error

Unexpected signal 11 when accessing memory at address (nil)
  | => tat _SM36scala.scalanative.unsafe.Tag$UShort$D4loadR_L16java.lang.ObjectEO
        at _SM50scala.scalanative.posix.sys.socketOps$sockaddrOps$D19sa_family$extensionL28scala.scalanative.unsafe.PtrL33scala.scalanative.unsigned.UShortEO
        at _SM26java.net.NetworkInterface$D7found$1LAb_iL28scala.scalanative.unsafe.PtrzEPT26java.net.NetworkInterface$
        ...

Adding null check so found returns false and lookup continues to next ifaddrs record.

[WojciechMazur]

The failing CI jobs are not related to this PR and can be ignored. These problems might be observed also in other PRs. I'm working on getting rid of these issues.