JPEG2000Stream.cc: Remove useless two if (priv->image)

We're already inside an if (priv->image) check
7 jobs for remove_useless_if in 8 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #7495458
build

00:05:42

passed #7495463
build_android

00:00:58

passed #7495459
build_clang_libcpp

00:08:12

passed #7495462
build_clazy

00:04:46

passed #7495461
build_mingw64_fedora

00:04:00

passed #7495460
build_ubuntu_16_04

00:02:41

passed #7495457
clang_format

00:01:35