value-initialise the buffer array member of detail::ansi_color_escape for constexpr use

[ishani]

I am using make_foreground_color<> in some code to generate the colouring escape sequence upfront (I realise I'm using things in client code from the detail namespace which might preclude me from asking for this to be fixed :) )

eg,

static constexpr auto foreground1 = fmt::detail::make_foreground_color<char>( _fg1 );

MSVC will refuse to compile this without the included patch, due to incomplete initialisation of the members in ansi_color_escape

... error C2131: expression did not evaluate to a constant
... failure was caused by a read of an uninitialized symbol
... see usage of 'fmt::v12::detail::ansi_color_escape<char>::buffer'

adding simple value-init to the buffer member satisfies the compiler. Does not change the outcome of any existing code.

[vitaut]

Thank you