Resolve warnings in VC-WIN32 build, which allows to add /WX.

It's argued that /WX allows to keep better focus on new code, which
motivates its comeback...

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4721)
diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 48f01ff..c1db7df 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -199,7 +199,7 @@
     size_t i;
     for (i = 0; i < OSSL_NELEM(nid_list); i++) {
         if (nid_list[i].nid == nid)
-            return i + 1;
+            return (uint16_t)(i + 1);
     }
     return 0;
 }