MB-28633: Don't create a temp std::string in ScanContext::manageSeparator

When creating a DocKey (which is just a view on the underlying data)
in ScanContext::manageSeparator(), don't create a temporary
std::string. This is illegal as when the string goes out of scope
(after calling DocKey::make() at line 34) the resulting 'cKey2' will
be pointing at now-freed heap (assuming no small-string optimization), 
or invalid stack (with small-string optimization).

Change-Id: I055d0d52277974b25b328f5fc3ca96b6fba9b6fa
Reviewed-on: http://review.couchbase.org/90796
Reviewed-by: Trond Norbye <[email protected]>
Reviewed-by: Tim Bradgate <[email protected]>
Tested-by: Build Bot <[email protected]>

Author: daverigby

engines/ep/src/collections/scan_context.cc

@@ -32,7 +32,8 @@ bool ScanContext::manageSeparator(const ::DocKey& key) {
             // Make a new Collections::DocKey from cKey.getKey, which returns
             // <uid>:<new_sep>, thus getKey will return <new_sep>
             auto cKey2 = Collections::DocKey::make(
-                    {{cKey.getKey().data(), cKey.getKey().size()},
+                    {reinterpret_cast<const uint8_t*>(cKey.getKey().data()),
+                     cKey.getKey().size(),
                      DocNamespace::System});
             separator.assign(cKey2.getKey().data(), cKey2.getKey().size());
             return true; // return true because we've changed