[CVE-2009-5155] Diagnose ERE '()|\1'

[CVE-2009-5155] Diagnose ERE '()|\1'

Problem reported by Hanno Böck in: http://bugs.gnu.org/21513
* lib/regcomp.c (parse_reg_exp): While parsing alternatives, keep
track of the set of previously-completed subexpressions available
before the first alternative, and restore this set just before
parsing each subsequent alternative.  This lets us diagnose the
invalid back-reference in the ERE '()|\1'.

Change-Id: I6de4f8c79837656f670b5c34a0869619af198abe
Signed-off-by: JinWang An <jinwang.an@samsung.com>