fixed invalid memory accesses in the expression stack and reworked expression stack

this was probably a regression from d94b18819ad4ee3237c46ad43a962d0121f0c3fe
and should not be in v0.5.
The return value of Expressions::find_op() must always be checked since
it might not find the operator, returning 0 (it used to be 0).
A zero index pointed to uninitialized memory - in the worst case it
pointed to invalid memory resulting in segfaults.
Too large indices were also not handled.
This was probably responsible for recent PPA build issues.
Valgrind/memcheck reports this error but I misread it as a bogus warning.

I took the opportunity to clean up the ValueStack implementation and
made it more robust by adding a few assertions.
ValueStacks now grow from large to small addresses (like stack data
structures usually do).
This means, there is no need to work with negative indices into the
stack pointer.
To reduce the potential for invalid stack accesses, stack indices are
now unsigned and have origin 0. Previously, all indices < 1 were
faulty but weren't checked.
Also, I added some minor optimizations.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/search.cpp b/src/search.cpp
index 370dbfc..8013385 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -527,7 +527,7 @@ StateSearch::done(const gchar *str)
 	if (eval_colon())
 		expressions.push(search_reg->get_integer());
 	else if (IS_FAILURE(search_reg->get_integer()) &&
-		 !expressions.find_op(Expressions::OP_LOOP) /* not in loop */)
+		 expressions.find_op(Expressions::OP_LOOP) < 0 /* not in loop */)
 		interface.msg(InterfaceCurrent::MSG_ERROR, "Search string not found!");
 
 	return &States::start;