libstdc++/regex: Zero-initialize _ExecutorFrame flags

libstdc++/regex: Zero-initialize _ExecutorFrame flags

Prevents uninitialized read bugs, and it's also just natural to assume
flags are initially cleared rather than uninitialized.

In passing rename the _M_end flag to _M_subexpr_end to make it distinct
from _Executor::_M_end.

libstdc++-v3/ChangeLog:

	* include/bits/regex_executor.tcc (_ExecutorFrameBase):
	Zero-initialize _M_byte0.
	(_ExecutorFrameBase::_M_end): Rename to ...
	(_ExecutorFrameBase::_M_subexpr_end): ... this.
	(_Executor::_M_handle_subexpr_begin): Remove now redundant
	clearing of _M_subexpr_end.
	(_Executor::_M_handle_subexpr_end): Adjust after renaming.
	(_Executor::_M_dfs) <case _S_fopcode_restore_cur_results>:
	Likewise.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>