Fix bug in Uppity RSA kex with short secret values.

In ssh_rsakex_decrypt, the code that decodes the buffer after it's
been through RSA decryption and had the OAEP masking undone would
never have worked if there were any padding 0 bytes between the prefix
and suffix of the OAEP preimage.

I must have not noticed before because PuTTY's RSA kex client code
always makes the biggest possible secret integer, so it never _does_
need any padding!

sshrsa.c

@@ -998,7 +998,7 @@ mp_int *ssh_rsakex_decrypt(
         if (out[i] == 1) {
             i++;  /* skip over the 1 byte */
             break;
-        } else if (out[i] != 1) {
+        } else if (out[i] != 0) {
             sfree(out);
             return NULL;
         }