poppler-page: minor optimization

on poppler_page_get_annot_mapping().
Let's retrieve the cropbox just once,
on the outside of the _for_ loop.
6 jobs for BUG_support_cropbox_in_annots in 11 minutes and 2 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #850600
build

00:04:18

passed #850605
build_android

00:01:07

passed #850601
build_clang_libcpp

00:03:53

passed #850604
build_clazy

00:03:08

passed #850603
build_mingw64_fedora

00:02:20

passed #850602
build_ubuntu_16_04

00:11:01