Auto merge of #43318 - jhjourdan:jh/fix_weak_cound_MAX, r=alexcrichton
Auto merge of #43318 - jhjourdan:jh/fix_weak_cound_MAX, r=alexcrichton

Fix in weak_count in Arc in the case the weak count is locked.

In the case the weak count was locked, the weak_count function could
return usize::MAX. We need to test this condition manually.

graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust