Silence obstack.c -Wc++compat warning

Silence obstack.c -Wc++compat warning

Fixes
warning: request for implicit conversion from ‘void *’ to ‘struct _obstack_chunk *’ not permitted in C++ [-Wc++-compat]

I moved the assignment to h->chunk to fix an overlong line, then
decided it would be better after the alloc failure check just to do
things the same way as in _obstack_newchunk.

	* obstack.c (_obstack_newchunk): Silence -Wc++compat warning.
	(_obstack_begin_worker): Likewise.  Move assignment to h->chunk
	after alloc failure check.