ssl_write_server_key_exchange: don't hard-code max hash size

library/ssl_srv.c

diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 2034d75..bc94ebb 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -3032,7 +3032,7 @@
     {
         size_t signature_len = 0;
         unsigned int hashlen = 0;
-        unsigned char hash[64];
+        unsigned char hash[MBEDTLS_MD_MAX_SIZE];
 
         /*
          * 3.1: Choose hash algorithm: