When backpatching a forward GOTO label, the old code erroneously
used CURSOR/HANDLER difference between context frames "c" and "a" to tune
a cpop/hpop command. So the cpop/hpop command later tried to pop
all cursors/handlers declared between "a" and "c", but those between
"b" and "c" were not cpushed/hpoped yet during the execution of "GOTO x".
Fixing the code to use the difference between frames "b" and "a" only.
BEGIN -- a
...
GOTO x; -- b
...
<<x>> -- c
...
END -- d
