BUILD: plock: remove dead code that causes a warning in gcc 11

BUILD: plock: remove dead code that causes a warning in gcc 11

As Ilya reported in issue #998, gcc 11 complains about misleading code
indentation which is in fact caused by dead assignments to zero after
a loop which stops on zero. Let's clean both of these.

(cherry picked from commit b1f54925fc17b00ddbe25e30c2207a8d68d47f34)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 775c665f20c695ad92fdb4318d5c77097ba15d7a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 78c677589e423f703cd9f5caefb28ed23bfba2a1)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>