busybox

dc: execute shouldn't pop if stack head is not a string

This matches the behaviour of both GNU dc (as specified in
its man page), and BSD dc (where stack_popstring() pops
only if the head is a string.)

Add a couple of tests to verify this behavior.

function                                             old     new   delta
zxc_vm_process                                      6882    6884      +2

Signed-off-by: Brian Foley <bpfoley@google.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>