Enforce and explicit some const casting

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1300)
diff --git a/ssl/t1_trce.c b/ssl/t1_trce.c
index 562ac09..ab6cfa2 100644
--- a/ssl/t1_trce.c
+++ b/ssl/t1_trce.c
@@ -728,7 +728,7 @@
         break;
 
     default:
-        BIO_dump_indent(bio, (char *)ext, extlen, indent + 2);
+        BIO_dump_indent(bio, (const char *)ext, extlen, indent + 2);
     }
     return 1;
 }
@@ -1257,7 +1257,7 @@
     default:
         BIO_indent(bio, indent + 2, 80);
         BIO_puts(bio, "Unsupported, hex dump follows:\n");
-        BIO_dump_indent(bio, (char *)msg, msglen, indent + 4);
+        BIO_dump_indent(bio, (const char *)msg, msglen, indent + 4);
     }
     return 1;
 }