Memory leak was caused by prepared statement fields metadata allocated
on MYSQL's field_alloc. field_alloc is reset by free_old_query()
function but only if mysql->fields is not NULL. In this case memory
was allocated by mysql_stmt_prepare() and then free_old_query() was
called from mysql_execute() while mysql->fields is NULL. Thus
mysql->field_alloc was reset without freeing allocated memory.

Fixed by extending cli_read_metadata() function to be able to accept
MEM_ROOT pointer. Extended function is cli_read_metadat_ex().
cli_read_metadata() is now simple shell which uses &mysql->field_alloc
as a parameter to cli_read_metadata_ex() . cli_read_prepare_result()
now uses stmt->mem_root to allocate memory for fields metadata. By the
way stmt->mem_root was used in this context before WL#7766 (commit
03149053bf7fb93e841fd68414109b69a4413018).

Reviewed-by: Rafal Somla <rafal.somla@oracle.com>
Reviewed-by: Praveen Hulakund <praveenkumar.hulakund@oracle.com>
RB: 8156