OSV-2020-843 Heap-buffer-overflow in uni_utf8_get_char_n

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23513 Crash type: Heap-buffer-overflow READ 1 Crash state: uniutf8getcharn smtpcommandparseparameters smtpcommandparseline...