io.c (fptr_finalize): free memory before GC sweep

This releases memory on explict calls to rb_io_close,
reducing pressure on the GC.

Final massif snapshot shows reduced heap usage after RubyGems load
(valgrind --tool=massif ./ruby -e exit)

before:
	mem_heap_B=4821992
	mem_heap_extra_B=1302952

after:
	mem_heap_B=4791056
	mem_heap_extra_B=1192440

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47758 b2dd03c8-39d4-4d8f-98ff-823fe69b080e