[Bug libstdc++/59531] string_view overrun in copy operation

[3dw4rd at verizon dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59531

--- Comment #1 from Ed Smith-Rowland <3dw4rd at verizon dot net> ---
Also, we should throw when pos > size() rather than pos >= size().
Spinning new patches and testing.