Remove a redundant failure check after an snew.

I spotted this at some point during this week's BinarySink
refactoring, but only just remembered to come back and fix it. snew
aborts the whole program rather than return NULL, so there's no need
to check its return value against NULL.

import.c

@@ -725,10 +725,6 @@ struct ssh2_userkey *openssh_pem_read(const Filename *filename,
 
         /* Construct the key */
         retkey = snew(struct ssh2_userkey);
-        if (!retkey) {
-            errmsg = "out of memory";
-            goto error;
-        }
         retkey->alg = alg;
 
         put_stringz(blob, alg->name);