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/handshake_helper.c b/test/handshake_helper.c
index ad1b709..188ec9e 100644
--- a/test/handshake_helper.c
+++ b/test/handshake_helper.c
@@ -350,14 +350,14 @@
         if ((*out)[i] == ',') {
             if (!TEST_int_gt(i - 1, prefix))
                 goto err;
-            (*out)[prefix] = i - 1 - prefix;
+            (*out)[prefix] = (unsigned char)(i - 1 - prefix);
             prefix = i;
         }
         i++;
     }
     if (!TEST_int_gt(len, prefix))
         goto err;
-    (*out)[prefix] = len - prefix;
+    (*out)[prefix] = (unsigned char)(len - prefix);
     return 1;
 
 err: