Fix two bugs in X509_REQ_add_extensions_nid(3)

Fix two bugs in X509_REQ_add_extensions_nid(3)
that i noticed while documneting the function:
* missing return value check for ASN1_item_i2d(3) and
* missing return value check for OBJ_nid2obj(3).

In the function X509_REQ_add_extensions_nid(3), merge everything
that is worth merging from the OpenSSL 1.1.1 branch, which is still
under a free license; that's mostly parts of the commit 9b0a4531
Mar 14 23:48:47 2015 +0000 (containing the bugfix, even though the
OpenSSL commit message did not mention the bugs) and some minor
stylistic changes from 0f113f3e and 26a7d938.

While here, use i2d_X509_EXTENSIONS(3) instead of the layer-violating
call to ASN1_item_i2d(3), and include a few stylistic tweaks from tb@.

OK tb@, and jsing@ agreed on the general direction.