bug#21513: assertion error in pop_fail_stack

[Hanno Böck]

Hi,

This command
grep -E '0|()0|\1|0'
will cause an assert error:
grep: regexec.c:1401: pop_fail_stack: Assertion `num >= 0' failed.


Happens both in the latest release (2.21) and the latest git code.

(this was found with american fuzzy lop)

cu,
-- 
Hanno Böck
http://hboeck.de/

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