Pageant: verify SSH-1 RSA keys before accepting them.

In Friday's testing of the BinarySink work, I noticed that if you
accidentally add a mathematically invalid RSA1 key to Pageant, it will
accept it, getting into a state where it can fail assertions when
asked to use the key later. Added a call to rsa_verify(), triggering
an SSH_AGENT_FAILURE response if it doesn't agree the key is good.

pageant.c

@@ -487,6 +487,11 @@ void pageant_handle_msg(BinarySink *bs,
 		goto add1_cleanup;
 	    }
 
+            if (!rsa_verify(key)) {
+                pageant_failure_msg(bs, "key is invalid", logctx, logfn);
+		goto add1_cleanup;
+            }
+
 	    comment = snewn(commentlen+1, char);
 	    if (comment) {
 		memcpy(comment, p + 4, commentlen);