OSV-2020-794 Use-of-uninitialized-value in Splash::pipeRunSimpleXBGR8

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23086 Crash type: Use-of-uninitialized-value Crash state: Splash::pipeRunSimpleXBGR8 Splash::blitImage Splash::drawImage...