OSV-2020-2303 Use-of-uninitialized-value in Splash::pipeRunSimpleXBGR8

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=29651 Crash type: Use-of-uninitialized-value Crash state: Splash::pipeRunSimpleXBGR8 Splash::drawSpan Splash::fillWithPattern...