fix build warnings with type mismatches and base check helpers

Building with base headers in a SLOT-ed setup exposes build warnings
(which causes failures due to -Werror).  One such example:

In file included from extent_writer.h:9:0,
                 from bzip_extent_writer.h:10,
                 from bzip_extent_writer.cc:5:
.../base/logging.h: In function 'std::string* logging::CheckEQImpl(const t1&, const t2&, const char*)
                                 [with t1 = unsigned int, t2 = int, std::string = std::basic_string<char>]':
bzip_extent_writer.cc:53:7:   instantiated from here
.../base/logging.h:512:1: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
cc1plus: all warnings being treated as errors

Explicitly cast the constants to avoid these.

BUG=chromium-os:16623
TEST=`emerge-x86-alex update_engine` builds

Change-Id: If3cc4e85fa54862b14305f9d045c73b5575efaa0
Reviewed-on: https://gerrit.chromium.org/gerrit/16035
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
diff --git a/cycle_breaker.cc b/cycle_breaker.cc
index 266c2df..8400366 100644
--- a/cycle_breaker.cc
+++ b/cycle_breaker.cc
@@ -87,7 +87,8 @@
 
 void CycleBreaker::HandleCircuit() {
   stack_.push_back(current_vertex_);
-  CHECK_GE(stack_.size(), 2);
+  CHECK_GE(stack_.size(),
+           static_cast<std::vector<Vertex::Index>::size_type>(2));
   Edge min_edge = make_pair(stack_[0], stack_[1]);
   uint64_t min_edge_weight = kuint64max;
   size_t edges_considered = 0;