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 3 minutes and 24 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1135318
build

00:03:24

passed #1135323
build_android

00:00:44

passed #1135319
build_clang_libcpp

00:03:08

passed #1135322
build_clazy

00:03:00

passed #1135321
build_mingw64_fedora

00:02:30

passed #1135320
build_ubuntu_16_04

00:01:51