Fix expansions that start from the end of another expansion (#839)

Do not free an expansion until its offset is *past* its size.
This means potentially freeing a nested stack of expansions
all at once.

Fixes #696

test/asm/interpolation-recursion.asm

@@ -1,6 +1,2 @@
-recurse EQUS "\{recurse\} "
+recurse EQUS "\{recurse\}"
 {recurse}
-
-; FIXME: also handle the following:
-; recurse EQUS "\{recurse\}"
-; {recurse}