[Bug libstdc++/78236] regex_iterator constructor is incomplete and creates uninitialized values that may be used

[christophe.monat at st dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78236

--- Comment #3 from Christophe Monat <christophe.monat at st dot com> ---
(In reply to Tim Shen from comment #2)
> I proposed another way to fix this in the list:
> https://gcc.gnu.org/ml/libstdc++/2016-11/msg00008.html

Looks perfect - I was somewhat annoyed by the _M_match() call anyway.

The test case is a reconstruction from a much bigger one exhibiting an issue
that we encountered at first on ARM/AArch64.