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/multi_range_http_fetcher.h b/multi_range_http_fetcher.h
index 19982ef..10829cc 100644
--- a/multi_range_http_fetcher.h
+++ b/multi_range_http_fetcher.h
@@ -46,7 +46,7 @@
   void ClearRanges() { ranges_.clear(); }
 
   void AddRange(off_t offset, size_t size) {
-    CHECK_GT(size, 0);
+    CHECK_GT(size, static_cast<size_t>(0));
     ranges_.push_back(Range(offset, size));
   }