shill: Fix a 'sign-compare' warning from clang.

BUG=chromium:386185
TEST=`P2_TEST_FILTER='shill::*' USE='clang asan' FEATURES=test emerge-$BOARD platform2`

Change-Id: I330ec198e14410dfb74b649c3b0abbfb689043b7
Reviewed-on: https://chromium-review.googlesource.com/204457
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Todd Broch <tbroch@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>

glib_io_input_handler.cc

diff --git a/glib_io_input_handler.cc b/glib_io_input_handler.cc
index 059d362..f8c678e 100644
--- a/glib_io_input_handler.cc
+++ b/glib_io_input_handler.cc
@@ -70,7 +70,7 @@
 
   if (status == G_IO_STATUS_EOF) {
     LOG(INFO) << "InputHandler on fd " << fd << " closing due to EOF.";
-    CHECK_EQ(len, 0);
+    CHECK_EQ(0U, len);
     return FALSE;
   }
   return TRUE;