fix fast_mp_invmod() not being able to handle case where b=1

This closes #67

bn_mp_invmod.c

@@ -25,7 +25,7 @@ int mp_invmod (mp_int * a, mp_int * b, mp_int * c)
 
 #ifdef BN_FAST_MP_INVMOD_C
   /* if the modulus is odd we can use a faster routine instead */
-  if (mp_isodd (b) == MP_YES) {
+  if ((mp_isodd(b) == MP_YES) && (mp_cmp_d(b, 1) != MP_EQ)) {
     return fast_mp_invmod (a, b, c);
   }
 #endif