Add missing initialization in bn/exponentiation

(Imported from upstream's 04f2a0b50d219aafcef2fa718d91462b587aa23d)

Change-Id: Ie840edeb1fc9d5a4273f137467e3ef16528c9668
Reviewed-on: https://boringssl-review.googlesource.com/7234
Reviewed-by: David Benjamin <davidben@google.com>

crypto/bn/exponentiation.c

@@ -209,6 +209,7 @@ static void BN_RECP_CTX_init(BN_RECP_CTX *recp) {
   BN_init(&recp->N);
   BN_init(&recp->Nr);
   recp->num_bits = 0;
+  recp->shift = 0;
   recp->flags = 0;
 }