ash: fix dequoting error detected by var_bash4 test

ash: fix dequoting error detected by var_bash4 test

But the test still fails (one more bug to fix)

function                                             old     new   delta
changepath                                           195     192      -3
readtoken1                                          3247    3240      -7
subevalvar                                          1204    1184     -20
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-30)             Total: -30 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>