MDEV-29473 UBSAN: Signed integer overflow: X * Y cannot be represented in type 'int' in strings/dtoa.c

Fixing a few problems relealed by UBSAN in type_float.test

- multiplication overflow in dtoa.c

- uninitialized Field::geom_type (and Field::srid as well)

- Wrong call-back function types used in combination with SHOW_FUNC.
  Changes in the mysql_show_var_func data type definition were not
  properly addressed all around the code by the following commits:
    b4ff6456
    18feb62f
    0ee879ff

  Adding a helper SHOW_FUNC_ENTRY() function and replacing
  all mysql_show_var_func declarations using SHOW_FUNC
  to SHOW_FUNC_ENTRY, to catch mysql_show_var_func in the future
  at compilation time.