ash: catch error in arithmetic expansion in PS1

ash: catch error in arithmetic expansion in PS1

Setting PS1 to:

   PS1='$((123+))'

causes the shell to enter an infinite error loop:

   sh: arithmetic syntax error

Catch any exception raised by expandarg() in expandstr() and allow
processing to continue.

function                                             old     new   delta
expandstr                                            262     344     +82
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 82/0)               Total: 82 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>