Make the fallback behavior of mbedtls_test_rnd_buffer_rand optional

If a fallback is not explicitly configured in the
mbedtls_test_rnd_buf_info structure, fail after the buffer is
exhausted.

There is no intended behavior change in this commit: all existing uses
of mbedtls_test_rnd_buffer_rand() have been updated to set
mbedtls_test_rnd_std_rand as the fallback.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_ecdsa.function b/tests/suites/test_suite_ecdsa.function
index 58cedc1..2cb8925 100644
--- a/tests/suites/test_suite_ecdsa.function
+++ b/tests/suites/test_suite_ecdsa.function
@@ -292,6 +292,8 @@
     TEST_ASSERT( mbedtls_mpi_read_string( &d, 16, d_str ) == 0 );
     TEST_ASSERT( mbedtls_mpi_read_string( &r_check, 16, r_str ) == 0 );
     TEST_ASSERT( mbedtls_mpi_read_string( &s_check, 16, s_str ) == 0 );
+    rnd_info.fallback_f_rng = mbedtls_test_rnd_std_rand;
+    rnd_info.fallback_p_rng = NULL;
     rnd_info.buf = rnd_buf->x;
     rnd_info.length = rnd_buf->len;