mesa: use default params for clearbuffer functions

this might otherwise pull in user-specified params
from the unpack buffer and access arbitrary data

cc: mesa-stable

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32945>

src/mesa/main/bufferobj.c

@@ -995,10 +995,11 @@ convert_clear_buffer_data(struct gl_context *ctx,
                           const GLvoid *data, const char *caller)
 {
    GLenum internalformatBase = _mesa_get_format_base_format(internalformat);
+   struct gl_pixelstore_attrib packing = {.Alignment = 1};
 
    if (_mesa_texstore(ctx, 1, internalformatBase, internalformat,
                       0, &clearValue, 1, 1, 1,
-                      format, type, data, &ctx->Unpack)) {
+                      format, type, data, &packing)) {
       return true;
    }
    else {