Remove some spurious null pointer tests.

In load_openssh_new_key, ret->keyblob is never null any more: now that
it's a strbuf instead of a bare realloc()ed string, it's at worst an
_empty_ strbuf. Secondly, as Coverity pointed out, the null pointer
check was too late to do any good in the first place - the previous
clause of the same if condition would already have dereferenced it!

In test_mac in the auxiliary testsc program, there's no actual reason
I would ever have called it with null ssh_mac pointer - it would mean
'don't test anything'! Looks as if I just copy-pasted the MAC parts of
the cipher+MAC setup code in test_cipher.

import.c

@@ -1166,7 +1166,7 @@ static struct openssh_new_key *load_openssh_new_key(const Filename *filename,
     fclose(fp);
     fp = NULL;
 
-    if (ret->keyblob->len == 0 || !ret->keyblob) {
+    if (ret->keyblob->len == 0) {
 	errmsg = "key body not present";
 	goto error;
     }