adminsocket: change failure test

This makes coverity happier:

CID 716941: Using invalid iterator (INVALIDATE_ITERATOR)
At (35): Dereferencing iterator "p" though it is already past the end of its container.

only because it doesn't understand all the side-effects of the failure
branch above.

Signed-off-by: Sage Weil <sage@inktank.com>

src/common/admin_socket.cc

@@ -331,7 +331,7 @@ bool AdminSocket::do_accept()
   }
 
   bufferlist out;
-  if (match.size() == 0) {
+  if (p == match.end()) {
     lderr(m_cct) << "AdminSocket: request '" << c << "' not defined" << dendl;
   } else {
     string args;