Fix GCC warning in `ssl_calc_finished_tls_sha384`

GCC 11 generated a warning because `padbuf` was too small to be
used as an argument for `mbedtls_sha512_finish_ret`. The `output`
parameter of `mbedtls_sha512_finish_ret` has the type
`unsigned char[64]`, but `padbuf` was only 48 bytes long.

Even though `ssl_calc_finished_tls_sha384` uses only 48 bytes for
the hash output, the size of `padbuf` was increased to 64 bytes.

Signed-off-by: Rodrigo Dias Correa <rodrigo@correas.us>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 725e9e7..8757e46 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -5411,7 +5411,7 @@
     int len = 12;
     const char *sender;
     mbedtls_sha512_context sha512;
-    unsigned char padbuf[48];
+    unsigned char padbuf[64];
 
     mbedtls_ssl_session *session = ssl->session_negotiate;
     if( !session )