d: Insert null terminator in obstack buffers

d: Insert null terminator in obstack buffers

Covers cases where functions that handle the extracted strings ignore
the explicit length.  This isn't something that's known to happen in the
current front-end, but the self-hosted front-end has been observed to do
this in its conversions between D and C-style strings.

gcc/d/ChangeLog:

	* d-lang.cc (deps_add_target): Insert null terminator in buffer.
	(deps_write): Likewise.
	(d_parse_file): Likewise.