Throw exception on allocation failure in cpp_regex_traits::value

[Martin <martin.raiber@…>]

Sorry, I'm testing the correct handling of allocation failures currently.

STL streams have the annoying property, that they do not throw exceptions on allocation failures (std::bad_alloc). This can be changed by calling basic_stringstream::exceptions(std::ios::badbit). This is not done in cpp_regex_traits.hpp line 565, causing, in my case, a unrelated exception (invalid quantifier).

See also #8966 for the same thing in lexical_cast.