winedbg: Don't dereference possibly NULL thread pointer.

programs/winedbg/gdbproxy.c

@@ -112,11 +112,11 @@ struct gdb_context
 static void gdbctx_delete_xpoint(struct gdb_context *gdbctx, struct dbg_thread *thread,
                                  dbg_ctx_t *ctx, struct gdb_xpoint *x)
 {
-    struct dbg_process *process = thread->process;
+    struct dbg_process *process = gdbctx->process;
     struct backend_cpu *cpu = process->be_cpu;
 
     if (!cpu->remove_Xpoint(process->handle, process->process_io, ctx, x->type, x->addr, x->value, x->size))
-        ERR("%04x:%04x: Couldn't remove breakpoint at:%p/%x type:%d\n", process->pid, thread->tid, x->addr, x->size, x->type);
+        ERR("%04x:%04x: Couldn't remove breakpoint at:%p/%x type:%d\n", process->pid, thread ? thread->tid : ~0, x->addr, x->size, x->type);
 
     list_remove(&x->entry);
     HeapFree(GetProcessHeap(), 0, x);