Test handling of non-existent x509 certs

In commit f905cc99 a use of
uninitialized data was fixed based on a coverity report. It
turns out it was possible to trigger this issue by pointing
libvirt at non-existent certificate files, typically causing
a crash.

This adds a test case for that scenario. With the above
commit reverted, this new test case will crash with a SEGV.
With the fix applied, it passes, reporting a normal libvirt
error to the caller.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

tests/virnettlscontexttest.c

@@ -563,6 +563,8 @@ mymain(void)
     DO_CTX_TEST(true, "cacertchain-ctx.pem", servercertlevel3areq.filename, false);
     DO_CTX_TEST(false, "cacertchain-ctx.pem", clientcertlevel2breq.filename, false);
 
+    DO_CTX_TEST(false, "cacertdoesnotexist.pem", "servercertdoesnotexist.pem", true);
+
     testTLSDiscardCert(&cacertreq);
     testTLSDiscardCert(&cacert1req);
     testTLSDiscardCert(&cacert2req);