bignum: fix boundary condition in montgomery logic

It's not clear whether this inconsistency could lead to an actual
computation error, but it involved a BIGNUM being passed around the
montgomery logic in an inconsistent state. This was found using flags
-DBN_DEBUG -DBN_DEBUG_RAND, and working backwards from this assertion
in 'ectest';

ectest: bn_mul.c:960: BN_mul: Assertion `(_bnum2->top == 0) ||
(_bnum2->d[_bnum2->top - 1] != 0)' failed

(Imported from upstream's 3cc546a3bbcbf26cd14fc45fb133d36820ed0a75)

crypto/bn/exponentiation.c

@@ -692,6 +692,9 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
     for (i = 1; i < j; i++)
       r->d[i] = (~m->d[i]) & BN_MASK2;
     r->top = j;
+    /* Upper words will be zero if the corresponding words of 'm'
+     * were 0xfff[...], so decrement r->top accordingly. */
+    bn_correct_top(r);
   } else if (!BN_to_montgomery(r, BN_value_one(), mont, ctx)) {
     goto err;
   }