OSV-2020-1000 Heap-buffer-overflow in xls_isCellTooSmall

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=12619 Crash type: Heap-buffer-overflow READ 1 Crash state: xlsisCellTooSmall xlspreparseWorkSheet xlsparseWorkSheet...