patch 8.0.1430: crash when term_start() fails

Problem:    Crash when term_start() fails.
Solution:   Initialize winpty_err.
diff --git a/src/terminal.c b/src/terminal.c
index 0cff47a..0d6a957 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -3386,7 +3386,7 @@
     HANDLE	    jo = NULL;
     HANDLE	    child_process_handle;
     HANDLE	    child_thread_handle;
-    void	    *winpty_err;
+    void	    *winpty_err = NULL;
     void	    *spawn_config = NULL;
     garray_T	    ga_cmd, ga_env;
     char_u	    *cmd = NULL;