DoList: Fix bug causing malformed lists to be generated

While evaluating the program for `DoList` or `DoSub`, the scratch pad
could be expanded by the program, adding "junk" to the generated list.

Added code that restores the scratchpad after program evaluation if
necessary.

This bug was detected by the GitHub CI on Windows where it was causing
an assertion failure on the generated list.

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>

Author: c3d

src/list.cc

@@ -2438,7 +2438,11 @@ COMMAND_BODY(DoList)
                         return ERROR;
                 }
 
+                size_t scratch = rt.allocated();
                 program::run(prg, true);
+                size_t scratch_used = rt.allocated();
+                if (scratch_used > scratch)
+                    rt.free(scratch_used - scratch);
 
                 size_t after = rt.depth();
                 if (after < depth)
@@ -2530,7 +2534,11 @@ COMMAND_BODY(DoSubs)
                     if (!rt.push(lst->at(i + d)))
                         return ERROR;
 
+                size_t scratch = rt.allocated();
                 program::run(prg, true);
+                size_t scratch_used = rt.allocated();
+                if (scratch_used > scratch)
+                    rt.free(scratch_used - scratch);
 
                 size_t after = rt.depth();
                 if (after < depth)