Change buf size to a valid size

Change the size of `buf` to a valid hash size, in `ecdsa_prim_random()`

Author: Ron Eldor

tests/suites/test_suite_ecdsa.function

@@ -14,7 +14,7 @@ void ecdsa_prim_random( int id )
     mbedtls_ecp_point Q;
     mbedtls_mpi d, r, s;
     rnd_pseudo_info rnd_info;
-    unsigned char buf[66];
+    unsigned char buf[MBEDTLS_MD_MAX_SIZE];
 
     mbedtls_ecp_group_init( &grp );
     mbedtls_ecp_point_init( &Q );