Fix GCC warning in ssl_calc_finished_tls_sha384

This commit fixes the same warning fixed by baeedbf, but without
wasting RAM. By casting `mbedtls_sha512_finish_ret()`, `padbuf`
could be kept 48 bytes long without triggering any warnings.

Signed-off-by: Rodrigo Dias Correa <rodrigo@correas.us>

Author: rodrigo-dc

library/ssl_tls.c

@@ -3197,12 +3197,15 @@ static void ssl_calc_finished_tls_sha256(
 #endif /* MBEDTLS_SHA256_C */
 
 #if defined(MBEDTLS_SHA512_C)
+
+typedef int (*finish_sha384_t)(mbedtls_sha512_context*, unsigned char[48]);
+
 static void ssl_calc_finished_tls_sha384(
                 mbedtls_ssl_context *ssl, unsigned char *buf, int from )
 {
     int len = 12;
     const char *sender;
-    unsigned char padbuf[64];
+    unsigned char padbuf[48];
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     size_t hash_size;
     psa_hash_operation_t sha384_psa = PSA_HASH_OPERATION_INIT;
@@ -3255,8 +3258,14 @@ static void ssl_calc_finished_tls_sha384(
     MBEDTLS_SSL_DEBUG_BUF( 4, "finished sha512 state", (unsigned char *)
                    sha512.state, sizeof( sha512.state ) );
 #endif
+    /*
+     * For SHA-384, we can save 16 bytes by keeping padbuf 48 bytes long.
+     * However, to avoid stringop-overflow warning in gcc, we have to cast
+     * mbedtls_sha512_finish_ret().
+     */
+    finish_sha384_t finish = (finish_sha384_t)mbedtls_sha512_finish_ret;
+    finish( &sha512, padbuf );
 
-    mbedtls_sha512_finish_ret( &sha512, padbuf );
     mbedtls_sha512_free( &sha512 );
 #endif