Improve LEB128 support and test coverage.

Also some minor cosmetic changes and style fixes.

BUG=aomedia:1125

Change-Id: Ide24057736376accd2f9907ef87b5a725ab3f3c2
diff --git a/tools/obu_parser.cc b/tools/obu_parser.cc
index fe288f9..ab68d3b 100644
--- a/tools/obu_parser.cc
+++ b/tools/obu_parser.cc
@@ -220,7 +220,7 @@
     consumed += current_obu_length;
   }
 
-  if (obu_overhead_bytes) *obu_overhead_bytes = obu_overhead;
+  if (obu_overhead_bytes != nullptr) *obu_overhead_bytes = obu_overhead;
 
   return true;
 }