ext_packed_depth_stencil: initialize GLuint array

ext_packed_depth_stencil: initialize GLuint array

```
/home/okias/projects/collabora/piglit/tests/util/piglit-dispatch-gen.h:5413:22: error: ‘p’ may be used uninitialized [-Werror=maybe-uninitialized]
 5413 | #define glDrawPixels piglit_dispatch_glDrawPixels
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/okias/projects/collabora/piglit/tests/util/piglit-dispatch-gen.h:5413:22: note: by argument 5 of type ‘const void *’ to ‘void(GLsizei,  GLsizei,  GLenum,  GLenum,  const void *)’ {aka ‘void(int,  int,  unsigned int,  unsigned int,  const void *)’}
 5413 | #define glDrawPixels piglit_dispatch_glDrawPixels
/home/okias/projects/collabora/piglit/tests/spec/ext_packed_depth_stencil/errors.c:45:9: note: in expansion of macro ‘glDrawPixels’
   45 |         glDrawPixels(1, 1, GL_DEPTH_STENCIL_EXT, GL_UNSIGNED_INT, p);
      |         ^~~~~~~~~~~~
/home/okias/projects/collabora/piglit/tests/spec/ext_packed_depth_stencil/errors.c:43:16: note: ‘p’ declared here
   43 |         GLuint p[1];
      |                ^
```

Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/piglit/-/merge_requests/726>