Fix crash when reading dummy CUs.

Fix crash when reading dummy CUs.

Dummy CUs are used by the incremental linker to pre-allocate space
in the output file. They have a DWARF header but no contents.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_per_cu_data): Add comment.
	(load_cu): Handle dummy CUs.
	(dw2_do_instantiate_symtab, process_queuef): Ditto.
	(dwarf2_fetch_die_loc_sect_off, dwarf2_fetch_constant_bytes): Ditto.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-dummy-cu.S: New file.
	* gdb.dwarf2/dw2-dummy-cu.exp: New file.