hush: fix "(sleep 1; exit 3) & sleep 2; echo $?; wait $!; echo $?"

function                                             old     new   delta
process_wait_result                                  414     426     +12
builtin_wait                                         283     291      +8
run_list                                             974     978      +4
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 24/0)               Total: 24 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-misc/wait6.right b/shell/hush_test/hush-misc/wait6.right
new file mode 100644
index 0000000..12decc1
--- /dev/null
+++ b/shell/hush_test/hush-misc/wait6.right
@@ -0,0 +1,2 @@
+0
+3
diff --git a/shell/hush_test/hush-misc/wait6.tests b/shell/hush_test/hush-misc/wait6.tests
new file mode 100755
index 0000000..c237131
--- /dev/null
+++ b/shell/hush_test/hush-misc/wait6.tests
@@ -0,0 +1,6 @@
+# In bash, "wait $!" extracts correct exitcode even if bg task has already exited
+# It prints 0, then 3:
+(sleep 0; exit 3) & sleep 1
+echo $?
+wait $!
+echo $?