Fix several invalid memory accesses reported by AFL + ASAN - Closes #3889

[JavierJF]

As the title says.

AFL execution results for completion:

Because launch_tester.sh offer the option of using keep_comment or not, the two options were used for parallel testing.

With mysql-query_digests_keep_comment=1:

Summary stats
=============

       Fuzzers alive : 0
      Dead or remote : 6 (included in stats)
      Total run time : 1 days, 1 hours
         Total execs : 1132 millions
    Cumulative speed : 73773 execs/sec
       Pending paths : 0 faves, 1315 total
       Crashes found : 0 locally unique
Cycles without finds : 412/6/2/11/4/1
  Time without finds : 0

With mysql-query_digests_keep_comment=0:

Summary stats
=============

       Fuzzers alive : 0
      Dead or remote : 6 (included in stats)
      Total run time : 19 hours, 18 minutes
         Total execs : 1050 millions
    Cumulative speed : 90726 execs/sec
       Pending paths : 0 faves, 1153 total
       Crashes found : 0 locally unique
Cycles without finds : 141/9/5/1/0/1
  Time without finds : 0

Full fuzzer results:

launch_tests_output.tar.gz
launch_tests_output_kc.tar.gz

[JavierJF]

Retest this please.