gio/subprocess: Initialize pid variable to 0

The documentation for g_spawn_async_with_pipes_and_fds() states:

> If an error occurs, child_pid, stdin_pipe_out, stdout_pipe_out, and
> stderr_pipe_out will not be filled with valid values.

Before 2dc3a6f0c80e5a8f786369eee0c45bfe19b55f4f, the `child_pid`
argument was `self->pid`, and GObject zero-initializes structs. So
the pid field was properly initialized to zero.

After 2dc3a6f0c80e5a8f786369eee0c45bfe19b55f4f, however, the out
variable is now declared inside initable_init(), and it's unitialized.
So if g_spawn_async_with_pipes_and_fds() errors out, `pid` will have
trash value in it, and the following assertion will fail:

```
g_assert (success == (pid != 0));
```

Fix that by initializing the `pid` variable to zero. Add a test to
exercise the fail code path, and prevent errors like this in the
future.

gio/gsubprocess.c

@@ -305,7 +305,7 @@ initable_init (GInitable     *initable,
   gint *pipe_ptrs[3] = { NULL, NULL, NULL };
   gint pipe_fds[3] = { -1, -1, -1 };
   gint close_fds[3] = { -1, -1, -1 };
-  GPid pid;
+  GPid pid = 0;
 #ifdef G_OS_UNIX
   gint stdin_fd = -1, stdout_fd = -1, stderr_fd = -1;
 #endif

gio/tests/gsubprocess.c

@@ -1352,6 +1352,22 @@ test_terminate (void)
   g_object_unref (proc);
 }
 
+static void
+test_fail_initialization (void)
+{
+  GError *local_error = NULL;
+  GError **error = &local_error;
+  GSubprocess *proc;
+
+  proc = g_subprocess_new (G_SUBPROCESS_FLAGS_NONE,
+                           error,
+                           "thisprogramshouldnotexistprettyplease",
+                           NULL);
+
+  g_assert_null (proc);
+  g_assert_error (local_error, G_SPAWN_ERROR, G_SPAWN_ERROR_NOENT);
+}
+
 #ifdef G_OS_UNIX
 static void
 send_signal (gpointer user_data)
@@ -2244,6 +2260,7 @@ main (int argc, char **argv)
   g_test_add_func ("/gsubprocess/communicate/utf8/invalid", test_communicate_utf8_invalid);
   g_test_add_func ("/gsubprocess/communicate/nothing", test_communicate_nothing);
   g_test_add_func ("/gsubprocess/terminate", test_terminate);
+  g_test_add_func ("/gsubprocess/fail-initialization", test_fail_initialization);
   g_test_add_func ("/gsubprocess/env", test_env);
   g_test_add_func ("/gsubprocess/env/inherit", test_env_inherit);
   g_test_add_func ("/gsubprocess/cwd", test_cwd);