sstable: fix buffer overflow in TOC

boost::split() expects either a NUL terminated string or a proper container.
We give it neither.

Fix by wrapping the buffer in a string_view, which tells split() what size
the string is.

sstables/sstables.cc

@@ -329,7 +329,7 @@ future<> sstable::read_toc() {
                 throw malformed_sstable_exception("SSTable too big: " + to_sstring(size) + " bytes.");
             }
 
-            auto buf = bufptr.get();
+            std::experimental::string_view buf(bufptr.get(), size);
             std::vector<sstring> comps;
 
             boost::split(comps , buf, boost::is_any_of("\n"));