Use CRYPTO_store_u32_be in tls_cbc.cc

It's a little shorter.

Change-Id: Id4b35e5eabfb7cff6fc8d738ad9dd1d7e9f9dde4
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/85667
Reviewed-by: Rudolf Polzer <rpolzer@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/cipher/tls_cbc.cc

diff --git a/crypto/cipher/tls_cbc.cc b/crypto/cipher/tls_cbc.cc
index 0918e23..747743d 100644
--- a/crypto/cipher/tls_cbc.cc
+++ b/crypto/cipher/tls_cbc.cc
@@ -169,10 +169,7 @@
   // The bounds above imply |total_bits| fits in four bytes.
   size_t total_bits = ctx->Nl + (len << 3);
   uint8_t length_bytes[4];
-  length_bytes[0] = (uint8_t)(total_bits >> 24);
-  length_bytes[1] = (uint8_t)(total_bits >> 16);
-  length_bytes[2] = (uint8_t)(total_bits >> 8);
-  length_bytes[3] = (uint8_t)total_bits;
+  CRYPTO_store_u32_be(length_bytes, total_bits);
 
   // We now construct and process each expected block in constant-time.
   uint8_t block[SHA_CBLOCK] = {0};
@@ -264,10 +261,7 @@
   // The bounds above imply |total_bits| fits in four bytes.
   size_t total_bits = ctx->Nl + (len << 3);
   uint8_t length_bytes[4];
-  length_bytes[0] = (uint8_t)(total_bits >> 24);
-  length_bytes[1] = (uint8_t)(total_bits >> 16);
-  length_bytes[2] = (uint8_t)(total_bits >> 8);
-  length_bytes[3] = (uint8_t)total_bits;
+  CRYPTO_store_u32_be(length_bytes, total_bits);
 
   // We now construct and process each expected block in constant-time.
   uint8_t block[SHA256_CBLOCK] = {0};