Remove unnecessary null pointer checks

[elfring]

An extra null pointer check is not needed in functions like the following.

• Buffer::DeallocHost
• Shape::Resize

[luotao1]

@elfring Thanks very much for this suggestion, you can create a PR to fix it.

[enkilee]

https://github.com/PaddlePaddle/CINN/pull/1140

[elfring]

:thought_balloon: Would you become interested to use a development tool like “clang-tidy” for corresponding source code adjustments?

[luotao1]

@elfring Yes, we have the plan to use clang-tidy, here is the plan. However we don't have enough people and time to start this plan.