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>

quote.c

@@ -471,7 +471,7 @@ char *nasm_skip_string(char *str)
 		break;
 	    }
 	}
-	return p;		/* Unterminated string... */
+	return p-1;		/* Unterminated string... */
     } else {
 	return str;		/* Not a string... */
     }