quote: Fix returning out of string pointer, take 2

In commit a45febd767 only part of problem has been covered.
Need to be ready for strings like

 | `a

http://bugzilla.nasm.us/show_bug.cgi?id=3392295

Reported-by: Hanno Boeck <hanno@hboeck.de>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>