OSV-2018-428 UNKNOWN READ in jbig2_decode_mmr_init

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=9681 Crash type: UNKNOWN READ Crash state: jbig2decodemmrinit jbig2decodegenericmmr jbig2decodesymboldict...