Check strings copied from another document for valid UTF-8

An editable document only holds valid UTF-8, but copy_scalar() copied the
strings and keys of a view of another document unchecked. A document of
a weaker check (or a borrowed text that changed after parsing) could
therefore bring ill-formed UTF-8 into it. The copy is checked now, with
the error that dump() reports for the string; copies within the same
document stay unchecked.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/edit.hpp

@@ -611,6 +611,8 @@ class editor
         switch (static_cast<value_t>(n.kind))
         {
             case value_t::string:
+                // (an editable document only holds valid UTF-8, whatever the check of the other document was)
+                check_utf8(from.str(n), n.len);
                 return string_node(from.str(n), n.len);
             case value_t::number_integer:
             case value_t::number_unsigned: