d: Fix D lexer sometimes fails to compile code read from stdin

d: Fix D lexer sometimes fails to compile code read from stdin

As of gdc-12, the lexer expects there 4 bytes of zero padding at the end
of the source buffer to mark the end of input.  Sometimes when reading
from stdin, the data at the end of input is garbage rather than zeroes.
Fix that by explicitly calling memset past the end of the buffer.

	PR d/105544

gcc/d/ChangeLog:

	* d-lang.cc (d_parse_file): Zero padding past the end of the stdin
	buffer so the D lexer has a sentinel to stop parsing at.