bn/bn_lib.c: remove bn_check_top from bn_expand2.

Trouble is that addition is postponing expansion till carry is
calculated, and if addition carries, top word can be zero, which
triggers assertion in bn_check_top.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NDavid Benjamin <davidben@google.com>
(Merged from https://github.com/openssl/openssl/pull/6662)

crypto/bn/bn_lib.c

@@ -209,8 +209,6 @@ static BN_ULONG *bn_expand_internal(const BIGNUM *b, int words)
 {
     BN_ULONG *a = NULL;
 
-    bn_check_top(b);
-
     if (words > (INT_MAX / (4 * BN_BITS2))) {
         BNerr(BN_F_BN_EXPAND_INTERNAL, BN_R_BIGNUM_TOO_LONG);
         return NULL;
@@ -245,8 +243,6 @@ static BN_ULONG *bn_expand_internal(const BIGNUM *b, int words)
 
 BIGNUM *bn_expand2(BIGNUM *b, int words)
 {
-    bn_check_top(b);
-
     if (words > b->dmax) {
         BN_ULONG *a = bn_expand_internal(b, words);
         if (!a)
@@ -259,7 +255,6 @@ BIGNUM *bn_expand2(BIGNUM *b, int words)
         b->dmax = words;
     }
 
-    bn_check_top(b);
     return b;
 }