Fix Fission (broken by my previous patch)

Fix Fission (broken by my previous patch)

Turns out somewhere along the refactoring of the multiple-CU support
for Fission I broke the patch before submitting it (& seems to have
broken Fission support generally).

Syncing back to the point at which the patch was committed, the
previous test results on my machine are:

 expected passes: 36137
 unexpected failures: 416

with the previous (broken) patch committed:

 expected passes: 36131
 unexpected failures: 429

With this one line patch applied on top of the broken commit:

 expected passes: 36144
 unexpected failures: 416

(& all other result counts remained the same in all 3 cases)

gdb/ChangeLog:
2017-07-18  David Blaikie  <dblaikie@gmail.com>

	* dwarf2read.c (create_cus_hash_table): Re-add lost initialization
	of dwo_cu's dwo_file.