[libc++][format] Fixes an off by one error.

The post-condition on the functions is that the buffer is not full.
This post-conditon is used as pre-condition of the push_back function.
When a copy, fill, of transform function exactly fit in the buffer this
post-condition was validated.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D155397