Enforce and explicit some const casting

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1300)

crypto/asn1/tasn_prn.c

@@ -423,7 +423,7 @@ static int asn1_print_obstring(BIO *out, ASN1_STRING *str, int indent)
     } else if (BIO_puts(out, "\n") <= 0)
         return 0;
     if ((str->length > 0)
-        && BIO_dump_indent(out, (char *)str->data, str->length,
+        && BIO_dump_indent(out, (const char *)str->data, str->length,
                            indent + 2) <= 0)
         return 0;
     return 1;

crypto/x509v3/v3_prn.c

@@ -188,7 +188,7 @@ static int unknown_ext_print(BIO *out, const unsigned char *ext, int extlen,
     case X509V3_EXT_PARSE_UNKNOWN:
         return ASN1_parse_dump(out, ext, extlen, indent, -1);
     case X509V3_EXT_DUMP_UNKNOWN:
-        return BIO_dump_indent(out, (char *)ext, extlen, indent);
+        return BIO_dump_indent(out, (const char *)ext, extlen, indent);
 
     default:
         return 1;

ssl/ssl_txt.c

@@ -128,7 +128,7 @@ int SSL_SESSION_print(BIO *bp, const SSL_SESSION *x)
     if (x->tlsext_tick) {
         if (BIO_puts(bp, "\n    TLS session ticket:\n") <= 0)
             goto err;
-        if (BIO_dump_indent(bp, (char *)x->tlsext_tick, x->tlsext_ticklen, 4)
+        if (BIO_dump_indent(bp, (const char *)x->tlsext_tick, x->tlsext_ticklen, 4)
             <= 0)
             goto err;
     }

ssl/t1_trce.c

@@ -728,7 +728,7 @@ static int ssl_print_extension(BIO *bio, int indent, int server, int extype,
         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 @@ static int ssl_print_handshake(BIO *bio, SSL *ssl,
     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;
 }