JBIG2Stream::readSymbolDictSeg: Return early if one of the bitmaps is null

Doesn't seem to regress any of the valid files i have and saves some
broken ones that loop for hours
7 jobs for 29254 in 5 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #6450936
build

00:05:31

passed #6450941
build_android

00:00:52

passed #6450937
build_clang_libcpp

00:05:32

passed #6450940
build_clazy

00:05:16

passed #6450939
build_mingw64_fedora

00:03:01

passed #6450938
build_ubuntu_16_04

00:02:44

passed #6450935
clang_format

00:01:32