From Matevz:
In TPRegexp::Compile() do not call Optimize() if regexp compilation has failed.

Add a static flag 'Bool_t TPRegexp::fgThrowAtCompileError' that allows callers
to intercept a faulty regexp being passed to Compile(). When this flag is
in effect, a failed regexp compilation throws a std::runtime_error.

Add member function 'Bool_t TPRegexp::IsValid() const' that returns true if
the object holds a valid compiled representation of the regexp.


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@48662 27541ba8-7e3a-0410-8455-c3a389f83636