Add init-free tests for RSA

These tests are trivial except when compiling with MBEDTLS_THREADING_C
and a mutex implementation that are picky about matching each
mbedtls_mutex_init() with exactly one mbedtls_mutex_free().

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/suites/test_suite_rsa.data

@@ -1,3 +1,9 @@
+RSA init-free-free
+rsa_init_free:0
+
+RSA init-free-init-free
+rsa_init_free:1
+
 RSA PKCS1 Verify v1.5 CAVS #1
 depends_on:MBEDTLS_SHA1_C:MBEDTLS_PKCS1_V15
 # Good padding but wrong hash

tests/suites/test_suite_rsa.function

@@ -17,6 +17,29 @@
  * END_DEPENDENCIES
  */
 
+/* BEGIN_CASE */
+void rsa_init_free( int reinit )
+{
+    mbedtls_rsa_context ctx;
+
+    /* Double free is not explicitly documented to work, but we rely on it
+     * even inside the library so that you can call mbedtls_rsa_free()
+     * unconditionally on an error path without checking whether it has
+     * already been called in the success path. */
+
+    mbedtls_rsa_init( &ctx, 0, 0 );
+    mbedtls_rsa_free( &ctx );
+
+    if( reinit )
+        mbedtls_rsa_init( &ctx, 0, 0 );
+    mbedtls_rsa_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 */
 void mbedtls_rsa_pkcs1_sign( char *message_hex_string, int padding_mode, int digest,
                      int mod, int radix_P, char *input_P, int radix_Q,