BUG/MAJOR: resolvers: segfault using server template without SRV RECORDs

BUG/MAJOR: resolvers: segfault using server template without SRV RECORDs

This patch fix the issue adding a test in srvrq before registering
the server on it during server template init.

This was a regression due to commit :
3406766d57fc11478d54a6fa2d048cbfe4524a4e

This should be backported with this previous commit (until 2.0)

(cherry picked from commit caef19e0c7594b00aa341d8984a33ddd0f15113f)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 3ecaf53374306519ff4394a7e7eebfe05235b9d6)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>