mds/Server.cc: fix dereference after null check

Add assert for diri->snaprealm before dereference it.

CID 716927 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "diri->snaprealm" to function
  "SnapRealm::resolve_snapname(std::string const &, inodeno_t,
   snapid_t, snapid_t)", which dereferences it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
.gitignore: add directory from coverity tools

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>