v3dv: avoid dereferencing null value

Fixes CID#1468079 "Dereference null return value (NULL_RETURNS)"

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10280>

src/broadcom/vulkan/v3dv_meta_clear.c

@@ -1516,6 +1516,9 @@ emit_tlb_clear(struct v3dv_cmd_buffer *cmd_buffer,
       v3dv_cmd_buffer_start_job(cmd_buffer, cmd_buffer->state.subpass_idx,
                                 V3DV_JOB_TYPE_GPU_CL);
 
+   if (!job)
+      return;
+
    /* vkCmdClearAttachments runs inside a render pass */
    job->is_subpass_continue = true;