MDEV-18046: Assortment of crashes, assertion failures and ASAN errors in mysql_show_binlog_events

Problem:
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SHOW BINLOG EVENTS FROM <pos> reports following assert when ASAN is enabled.

uint32 binlog_get_uncompress_len(const char*):
  Assertion `(buf[0] & 0xe0) == 0x80' failed

Fix:
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**Part11: Converted debug assert to error handler code**