v9.1.0 gcc6 compilation error: uninitialized variable in constexpr function

[glebm]

On GCC6, after fixing #3339, the build fails with:

include/fmt/format.h:1241:8: error: uninitialized variable 'buffer' in 'constexpr' function
   Char buffer[digits10<UInt>() + 1];
        ^~~~~~

https://app.circleci.com/pipelines/github/diasurgical/devilutionX/17078/workflows/eeb9881e-ed3d-46c3-93d5-239b5100e56e/jobs/108888 (diasurgical/DevilutionX#5875)

(extracted from #3339 (comment))

[phprus]

Already fixed in master:

fmt/include/fmt/format.h

Line 1345 in cbc7b8d

Char buffer[digits10<UInt>() + 1] = {};

[glebm]

Thanks! Fixed by #3066