glthread: handle DeleteBuffers(n=-1) gracefully

this is a valid error condition

cc: mesa-stable

fixes:
GTF-GL46.gtf21.GL2FixedTests.buffer_objects.buffer_objects

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18856>

src/mesa/main/glthread_bufferobj.c

@@ -210,7 +210,7 @@ _mesa_glthread_DeleteBuffers(struct gl_context *ctx, GLsizei n,
 {
    struct glthread_state *glthread = &ctx->GLThread;
 
-   if (!buffers)
+   if (!buffers || n < 0)
       return;
 
    for (unsigned i = 0; i < n; i++) {