Fix a mismerge in kex null-pointer checks.

Fix a mismerge in kex null-pointer checks.

I removed a vital line of code while fixing the merge conflicts when
cherry-picking 1eb578a488a71284d6b18e46df301e54805f2c35 as
26fe1e26c0f7ab42440332882295667d4a0ac500, causing Diffie-Hellman key
exchange to be completely broken because the server's host key was
never constructed to verify the signature with. Reinstate it.