get-array-attribs: prevent uninitialized array warning

get-array-attribs: prevent uninitialized array warning

```
/home/okias/projects/collabora/piglit/tests/util/piglit-dispatch-gen.h:14777:25: error: ‘dummy’ may be used uninitialized [-Werror=maybe-uninitialized]
14777 | #define glVertexPointer piglit_dispatch_glVertexPointer
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/okias/projects/collabora/piglit/tests/util/piglit-dispatch-gen.h:14777:25: note: by argument 4 of type ‘const void *’ to ‘void(GLint,  GLenum,  GLsizei,  const void *)’ {aka ‘void(int,  unsigned int,  int,  const void *)’}
14777 | #define glVertexPointer piglit_dispatch_glVertexPointer
/home/okias/projects/collabora/piglit/tests/spec/gl-1.5/get-array-attribs.c:98:9: note: in expansion of macro ‘glVertexPointer’
   98 |         glVertexPointer(2, GL_FLOAT, 12, dummy);
      |         ^~~~~~~~~~~~~~~
/home/okias/projects/collabora/piglit/tests/spec/gl-1.5/get-array-attribs.c:96:17: note: ‘dummy’ declared here
   96 |         GLubyte dummy[100];
      |                 ^~~~~
```

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>