Change function casting in `ssl_calc_finished_tls_sha384`

`finish_sha384_t` was made more generic by using `unsigned char*`
instead of `unsigned char[48]` as the second parameter.
This change tries to make the function casting more robust against
future improvements of gcc analysis.

Signed-off-by: Rodrigo Dias Correa <rodrigo@correas.us>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 70d32cc..7f0e075 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -5406,7 +5406,7 @@
 
 #if defined(MBEDTLS_SHA512_C)
 
-typedef int (*finish_sha384_t)(mbedtls_sha512_context*, unsigned char[48]);
+typedef int (*finish_sha384_t)(mbedtls_sha512_context*, unsigned char*);
 
 static void ssl_calc_finished_tls_sha384(
                 mbedtls_ssl_context *ssl, unsigned char *buf, int from )