Check obj1 is a stream before getting the stream

It seems we already did this check a few lines above, and indeed we did,
but on very broken documents, if arr[1] is a Ref, getting objects may
end up in a reconstruct xref call which may end up changing the type of
arr[1] the next time we ask for it

oss-fuzz/29260
7 jobs for more_fuzzying_stuff in 6 minutes and 52 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #6462501
build

00:05:01

passed #6462506
build_android

00:01:04

passed #6462502
build_clang_libcpp

00:06:52

passed #6462505
build_clazy

00:04:44

passed #6462504
build_mingw64_fedora

00:03:37

passed #6462503
build_ubuntu_16_04

00:02:22

passed #6462500
clang_format

00:01:57