Add init-free tests for entropy

These tests validate that an entropy object can be reused and that
calling mbedtls_entropy_free() twice is ok.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_entropy.function b/tests/suites/test_suite_entropy.function
index 3aee854..f4f9693 100644
--- a/tests/suites/test_suite_entropy.function
+++ b/tests/suites/test_suite_entropy.function
@@ -125,6 +125,28 @@
  * END_DEPENDENCIES
  */
 
+/* BEGIN_CASE */
+void entropy_init_free( int reinit )
+{
+    mbedtls_entropy_context ctx;
+
+    /* Double free is not explicitly documented to work, but it is convenient
+     * to call mbedtls_entropy_free() unconditionally on an error path without
+     * checking whether it has already been called in the success path. */
+
+    mbedtls_entropy_init( &ctx );
+    mbedtls_entropy_free( &ctx );
+
+    if( reinit )
+        mbedtls_entropy_init( &ctx );
+    mbedtls_entropy_free( &ctx );
+
+    /* This test case always succeeds, functionally speaking. A plausible
+     * bug might trigger an invalid pointer dereference or a memory leak. */
+    goto exit;
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_ENTROPY_NV_SEED:MBEDTLS_FS_IO */
 void entropy_seed_file( char * path, int ret )
 {