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.data b/tests/suites/test_suite_debug.data
index f1d29ef..4a1a1be 100644
--- a/tests/suites/test_suite_debug.data
+++ b/tests/suites/test_suite_debug.data
@@ -37,6 +37,9 @@
 Debug print buffer #5
 mbedtls_debug_print_buf:"MyFile":999:"Test return value":"000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F202122232425262728292A2B2C2D2E2F30":"MyFile(0999)\: dumping 'Test return value' (49 bytes)\nMyFile(0999)\: 0000\:  00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f  ................\nMyFile(0999)\: 0010\:  10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f  ................\nMyFile(0999)\: 0020\:  20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f   !"#$%&'()*+,-./\nMyFile(0999)\: 0030\:  30                                               0\n"
 
+Debug print mbedtls_mpi: 0 (empty representation)
+mbedtls_debug_print_mpi:16:"":"MyFile":999:"VALUE":"MyFile(0999)\: value of 'VALUE' (0 bits) is\:\nMyFile(0999)\:  00\n"
+
 Debug print mbedtls_mpi: 0 (non-empty representation)
 mbedtls_debug_print_mpi:16:"00000000000000":"MyFile":999:"VALUE":"MyFile(0999)\: value of 'VALUE' (0 bits) is\:\nMyFile(0999)\:  00\n"
 
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);