tests/ssl_helpers: Check that message queue is popped

mbedtls_test_mock_tcp_recv_msg is currently popping a message
queue and does not check if this was done correctly.

This extra check makes the test more complete/robust.

Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
diff --git a/tests/src/test_helpers/ssl_helpers.c b/tests/src/test_helpers/ssl_helpers.c
index 255849f..16fa38a 100644
--- a/tests/src/test_helpers/ssl_helpers.c
+++ b/tests/src/test_helpers/ssl_helpers.c
@@ -551,7 +551,10 @@
              * happen in test environment, unless forced manually. */
         }
     }
-    mbedtls_test_ssl_message_queue_pop_info(queue, buf_len);
+    ret = mbedtls_test_ssl_message_queue_pop_info(queue, buf_len);
+    if (ret < 0) {
+        return ret;
+    }
 
     return (msg_len > INT_MAX) ? INT_MAX : (int) msg_len;
 }