Bytecode: fix stack_solve_algo=4 when a “solve backward/forward” block follows a “solve two boundaries” block

When solving a “Solve two boundarise” block with stack_solve_algo=4, the
“slowc” variable is modified. This would affect the resolution of further
“solve backward/forward” blocks, which would yield results.

The fix consists in saving and restoring “slowc”.

(cherry picked from commit 96fb676b)