BUG/MINOR: debug: fix pointer check in debug_parse_cli_task()

BUG/MINOR: debug: fix pointer check in debug_parse_cli_task()

Task pointer check in debug_parse_cli_task() computes the theoric end
address of provided task pointer to check if it is valid or not thanks to
may_access() helper function.

However, relative ending address is calculated by adding task size to 't'
pointer (which is a struct task pointer), thus it will result to incorrect
address since the compiler automatically translates 't + x' to
't + x * sizeof(*t)' internally (with sizeof(*t) != 1 here).

Solving the issue by using 'ptr' (which is the void * raw address) as
starting address to prevent automatic address scaling.

This was revealed by coverity, see GH #2157.

No backport is needed, unless 9867987 ("DEBUG: cli: add "debug dev task"
to show/wake/expire/kill tasks and tasklets") gets backported.