PR bootstrap/55380
PR other/54691
* files.c (read_file_guts): Allocate extra 16 bytes instead of
1 byte at the end of buf. Pass size + 16 instead of size
to _cpp_convert_input.
* charset.c (_cpp_convert_input): Reallocate if there aren't
at least 16 bytes beyond to.len in the buffer. Clear 16 bytes
at to.text + to.len.
From-SVN: r194102