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/test/asn1_encode_test.c b/test/asn1_encode_test.c
index 3b8d418..e9f459a 100644
--- a/test/asn1_encode_test.c
+++ b/test/asn1_encode_test.c
@@ -577,14 +577,14 @@
 
     if (pp != NULL) {
         if (lenbytes == 1) {
-            *(*pp)++ = len;
+            *(*pp)++ = (unsigned char)len;
         } else {
-            *(*pp)++ = lenbytes - 1;
+            *(*pp)++ = (unsigned char)(lenbytes - 1);
             if (lenbytes == 2) {
-                *(*pp)++ = 0x80 | len;
+                *(*pp)++ = (unsigned char)(0x80 | len);
             } else {
-                *(*pp)++ = 0x80 | (len >> 8);
-                *(*pp)++ = len & 0xff;
+                *(*pp)++ = (unsigned char)(0x80 | (len >> 8));
+                *(*pp)++ = (unsigned char)(len);
             }
         }
     }