main/asterisk.c: ast_el_read_char

Make sure buf[res] is not accessed at res=-1 (buffer underrun).
Address Sanitizer will complain about this quite loudly.

ASTERISK-24801 #close

Change-Id: Ifcd7f691310815a31756b76067c56fba299d3ae9

main/asterisk.c

@@ -2808,13 +2808,14 @@ static int ast_el_read_char(EditLine *editline, char *cp)
 
 			console_print(buf, 0);
 
-			if ((res < EL_BUF_SIZE - 1) && ((buf[res-1] == '\n') || (buf[res-2] == '\n'))) {
+			if ((res < EL_BUF_SIZE - 1) && ((buf[res-1] == '\n') || (res >= 2 && buf[res-2] == '\n'))) {
 				*cp = CC_REFRESH;
 				return(1);
-			} else
+			} else {
 				lastpos = 1;
 			}
 		}
+	}
 
 	*cp = '\0';
 	return (0);