If a struct contains an anonymous union and both have a field with the
same name, detect_field_duplicates_hash() will replace one of them
with NULL.  If compilation doesn't stop immediately, it may later call
lookup_field() on the union, which falsely assumes the union's
LANG_SPECIFIC array is sorted, and may loop indefinitely because of
this.

2018-08-03  Bogdan Harjoc  <harjoc@gmail.com>

	PR c/86690
gcc/c:
	* c-typeck.c (lookup_field): Do not use TYPE_LANG_SPECIFIC after
	errors.

gcc/testsuite:
	* gcc.dg/union-duplicate-field.c: New test.

From-SVN: r263294