Fix error handling in bn_exp

In the event of an error |rr| could be NULL. Therefore don't assume you can
use |rr| in the error handling code.

(Imported from upstream's 8c5a7b33c6269c3bd6bc0df6b4c22e4fba03b485.)

Change-Id: I0b392991ce8170dc418e93003af256d535d1e2e8
Reviewed-on: https://boringssl-review.googlesource.com/4005
Reviewed-by: Adam Langley <agl@google.com>

crypto/bn/exponentiation.c

@@ -172,12 +172,13 @@ int BN_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx) {
       }
     }
   }
-  ret = 1;
 
-err:
   if (r != rr) {
     BN_copy(r, rr);
   }
+  ret = 1;
+
+err:
   BN_CTX_end(ctx);
   return ret;
 }