ash: fix another bit of var_bash4 bug

But it _still_ doesn't pass! quoted case is a tough nut to crack

function                                             old     new   delta
redirect                                            1281    1286      +5
subevalvar                                          1141    1142      +1

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

shell/ash_test/ash-vars/var_bash4.tests

@@ -30,7 +30,7 @@ v='a\bc'
 echo 'Source:       ' "$v"
 echo 'Replace str:  ' '_\\_\z_'
 
-echo 'Pattern:      ' 'single backslash and b: "replace literal b"'
+echo 'Pattern:      ' 'single backslash and b: "replace b"'
 r=${v/\b/_\\_\z_}
 echo 'In assignment:' "$r"
 echo 'Unquoted:     '  ${v/\b/_\\_\z_}