Fix memory leak when exception is raised during JSON generation part 2

[rhenium]

Commit 44df509 fixed it for StandardError, but other exceptions and jumps are also possible. Use rb_ensure() to release FBuffer instead of rb_rescue().

A reproducer:

o = Object.new
def o.to_json(a) = throw :a
a = ["make heap allocation"*100, o]

10.times do
  100_000.times do
    catch(:a) { JSON(a) }
  end

  puts `ps -o rss= -p #{$$}`
end