__output_buffer::push_back writes a code unit before it makes room, so
the buffer must always have a free one. __copy, __transform and __fill
write in chunks of the available space and could end with the buffer
full. Formatting into a back_insert_iterator of a string, vector or
deque goes through a 256 code unit buffer, so an argument or fill whose
length is a multiple of 256, followed by a literal, wrote that literal
past the end of the buffer.
Make room in a full buffer at the end of these functions, as push_back
does.
Fixes#154670
(cherry picked from commit 372dc97670f7b3ce422dc26e5f626b2711426e8d)
ykiko
31c414240a