Add mbedtls_debug_print_mpi test case for 0

There was already a test case for 0 but with a non-empty representation
(X->n == 1). Add a test case with X->n == 0 (freshly initialized mpi).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_debug.function b/tests/suites/test_suite_debug.function
index ad50e53..fda6939 100644
--- a/tests/suites/test_suite_debug.function
+++ b/tests/suites/test_suite_debug.function
@@ -179,7 +179,9 @@
 
     TEST_ASSERT( mbedtls_ssl_setup( &ssl, &conf ) == 0 );
 
-    TEST_ASSERT( mbedtls_mpi_read_string( &val, radix, value ) == 0 );
+    /* If value is empty, keep val->n == 0. */
+    if( value[0] != 0 )
+        TEST_ASSERT( mbedtls_mpi_read_string( &val, radix, value ) == 0 );
 
     mbedtls_ssl_conf_dbg( &conf, string_debug, &buffer);