ash: refresh stack pointers after makestrspace in rmescapes

ash: refresh stack pointers after makestrspace in rmescapes

Without this, rmescapes sometimes returns random garbage while
performing parameter expansions such as ${foo#bar}, in the event that
the allocation of r happens to need to reallocate the stack and hence
invalidate str and p.  I'd love to provide a test case but unfortunately
it's dependent on exact stack layout, so I don't have anything simpler
than the situation described in
https://bugs.launchpad.net/ubuntu/+source/partman-base/+bug/527401/comments/23
which involved a sequence of foo="${foo#*, }" expansions on a long
string inside our RAID configuration tool.

The same fix has been in dash since 2007-09-26, contributed by Roy
Marples <uberlord@gentoo.org>.  I actually came up with it independently
almost to the character, but then synced it up with the variable naming
used in dash when I noticed that change afterwards.

Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>