vk/cmd_buffer: Initialize the pipeline pointer to NULL

If a meta operation is called before the pipeline is set, this can cause
uses of undefined values.  They *should* be harmless, but we might as well
shut up valgrind on this one too.

src/vulkan/device.c

@@ -2178,6 +2178,7 @@ VkResult anv_CreateCommandBuffer(
 
    cmd_buffer->dirty = 0;
    cmd_buffer->vb_dirty = 0;
+   cmd_buffer->pipeline = NULL;
 
    *pCmdBuffer = (VkCmdBuffer) cmd_buffer;