MDEV-31893 Valgrind reports issues in main.join_cache_notasan
This is also related to MDEV-31348 Assertion `last_key_entry >= end_pos' failed in virtual bool JOIN_CACHE_HASHED::put_record() Valgrind exposed a problem with the join_cache for hash joins: =25636== Conditional jump or move depends on uninitialised value(s) ==25636== at 0xA8FF4E: JOIN_CACHE_HASHED::init_hash_table() (sql_join_cache.cc:2901) The reason for this was that avg_record_length contained a random value if one had used SET optimizer_switch='optimize_join_buffer_size=off'. This causes either 'random size' memory to be allocated (up to join_buffer_size) which can increase memory usage or, if avg_record_length is less than the row size, memory overwrites in thd->mem_root, which is bad. Fixed by setting avg_record_length in JOIN_CACHE_HASHED::init() before it's used. There is no test case for MDEV-31893 as valgrind of join_cache_notasan checks that. I added a test case for MDEV-31348.
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