Fix a few memory leaks.

Mostly noticed in passing while using Address / Leak Sanitiser to
check over the previous commit. One highlight here is freeing of the
previous iqmp value in rsa_verify, which was actually a potentially
sensitive leak, introduced in the mp_int rewrite (commit 25b034ee3).

sshprime.c

@@ -451,6 +451,7 @@ void invent_firstbits(unsigned *one, unsigned *two, unsigned min_separation)
     }
 
     mp_free(randval);
+    mp_free(total);
 
     /*
      * Check everything came out right.