re PR bootstrap/55380 (All search_line_fast implementations read beyond buffer)

	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