[BusyBox] BB_FEATURE_SH_STANDALONE_SHELL broken in ash in 0.60.5 (was: add assert to detect infinite loop in findvar() )

[Dan Kegel]

Disabling BB_FEATURE_SH_STANDALONE_SHELL seems to prevent
the problem; I can then happily invoke ash from inside ash
without it hanging.  Wonder what change since 0.60.2 broke this?