Fix kernel version parsing failed too early.

[imkiva]

libbpf does not fail that early when get_{OS}_kernel_version() failed to scan kernel version strings from either /proc/version_signature or from utsname::version

https://github.com/libbpf/libbpf/blob/855bf91055a6250642dff1390231387cc5540711/src/libbpf_probes.c#L80-L100

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[tamird]

This is the same as #909. Like that one, this needs tests.

[alessandrod]

can this be closed now that #909 has been merged?