Splash: fix uninitialized memory read on broken files

oss-fuzz/23086
7 jobs for uninit_splash in 3 minutes and 39 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #6688416
build

00:03:14

passed #6688421
build_android

00:00:50

passed #6688417
build_clang_libcpp

00:03:22

passed #6688420
build_clazy

00:03:38

passed #6688419
build_mingw64_fedora

00:02:52

passed #6688418
build_ubuntu_16_04

00:02:19

passed #6688415
clang_format

00:00:54