glib/poppler-structure-element: fix memleak

in convert_doubles_array(), which was not
assigning the allocated memory to the
out variable.

This means this function was never returning
anything until now, which means there's probably
no users of this function in the wild (certainly
not in Evince).

Issue #1049
7 jobs for glib_mem_leak_convert_doubles_array in 4 minutes and 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #7646526
build

00:04:44

passed #7646531
build_android

00:00:51

passed #7646527
build_clang_libcpp

00:04:43

passed #7646530
build_clazy

00:04:17

passed #7646529
build_mingw64_fedora

00:02:45

passed #7646528
build_ubuntu_16_04

00:02:36

passed #7646525
clang_format

00:01:12