MDEV-28345 ASAN: use-after-poison or unknown-crash in my_strtod_int from charset_info_st::strntod or test_if_number

This patch fixes two problems:

- The code inside my_strtod_int() in strings/dtoa.c could test the byte
  behind the end of the string when processing the mantissa.
  Rewriting the code to avoid this.

- The code in test_if_number() in sql/sql_analyse.cc called my_atof()
  which is unsafe and makes the called my_strtod_int() look behind
  the end of the string if the input string is not 0-terminated.
  Fixing test_if_number() to use my_strtod() instead, passing the correct
  end pointer.