tests: Fix segfault with mbedTLS built without threading support

torture_rand and torture_server_x11 call ssh_init without checking
the return value. If mbedTLS is built without threading support
ssh_init fails but the tests continue and then segfault since threading
wasn't correctly initialised.

Add a section that documents requirements for mbedTLS usage in a
multi threaded environment to README.mbedtls.

Signed-off-by: Juraj Vijtiuk <juraj.vijtiuk@sartura.hr>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

README.mbedtls

@@ -1,3 +1,6 @@
+libssh mbedTLS ECDSA support
+=============================
+
 When built with mbedTLS, libssh currently does not support ECDSA key comparison.
 Since the comparison function is used during the verification of publickey
 authentication requests a libssh server will not be able to deal with ECDSA
@@ -5,3 +8,16 @@ keys.
 
 In general, if the ssh_key_cmp function is used with mbedTLS, ECDSA key
 comparison won't work.
+
+
+mbedTLS and libssh in multithreaded applications
+==================================================
+
+To use libssh with mbedTLS in a multithreaded application, mbedTLS has to be
+built with threading support enabled.
+
+If threading support is not available and multi threading is used, ssh_init
+will fail.
+
+More information about building mbedTLS with threading support can be found
+in the mbedTLS documentation.

tests/unittests/torture_rand.c

@@ -17,9 +17,13 @@
 
 static int setup(void **state) {
     (void) state;
+    int rc;
 
     ssh_threads_set_callbacks(ssh_threads_get_pthread());
-    ssh_init();
+    rc = ssh_init();
+    if (rc != SSH_OK) {
+        return -1;
+    }
 
     return 0;
 }

tests/unittests/torture_server_x11.c

@@ -21,9 +21,13 @@ struct hostkey_state {
 static int setup(void **state) {
     struct hostkey_state *h;
     mode_t mask;
+    int rc;
 
     ssh_threads_set_callbacks(ssh_threads_get_pthread());
-    ssh_init();
+    rc = ssh_init();
+    if (rc != SSH_OK) {
+        return -1;
+    }
 
     h = malloc(sizeof(struct hostkey_state));
     assert_non_null(h);