Auto merge of #44802 - sfackler:vecdeque-oob, r=Gankro
Fix capacity comparison in reserve
You can otherwise end up in a situation where you don't actually resize
but still call into handle_cap_increase which then corrupts head/tail.
Closes #44800
Not totally sure the right way to write a test for this - there are some debug asserts the old bad behavior will hit but we don't build the stdlib with debug assertions by default.
r? @Gankro