[libc++] Fix segmentation fault in __do_put_integral

6 chars are not sufficient to represent all formats for 64 bit integers.

This was accidentally introduced in commit b889cbf3 (https://reviews.llvm.org/D112830).

This causes failures in downstream projects, for example:

* https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=40817
* https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=40841

Differential Revision: https://reviews.llvm.org/D113600