Some small cleaning up to make some compiling environment happy

Summary: Compiler complains some errors when building using our internal build settings. Fix them.

Test Plan: rebuild

Reviewers: haobo, dhruba, igor, yhchiang, ljin

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17199

db/corruption_test.cc

@@ -387,7 +387,7 @@ TEST(CorruptionTest, FileSystemStateCorrupted) {
     DBImpl* dbi = reinterpret_cast<DBImpl*>(db_);
     std::vector<LiveFileMetaData> metadata;
     dbi->GetLiveFilesMetaData(&metadata);
-    ASSERT_GT(metadata.size(), 0);
+    ASSERT_GT(metadata.size(), size_t(0));
     std::string filename = dbname_ + metadata[0].name;
 
     delete db_;

db/merge_operator.cc

@@ -23,7 +23,7 @@ bool MergeOperator::PartialMergeMulti(const Slice& key,
   std::string temp_value;
   Slice temp_slice(operand_list[0]);
 
-  for (int i = 1; i < operand_list.size(); ++i) {
+  for (size_t i = 1; i < operand_list.size(); ++i) {
     auto& operand = operand_list[i];
     if (!PartialMerge(key, temp_slice, operand, &temp_value, logger)) {
       return false;

util/env_test.cc

@@ -481,9 +481,9 @@ class TestLogger : public Logger {
 
       if (new_format[0] == '[') {
         // "[DEBUG] "
-        ASSERT_TRUE(n <= 56 + (512 - sizeof(struct timeval)));
+        ASSERT_TRUE(n <= 56 + (512 - static_cast<int>(sizeof(struct timeval))));
       } else {
-        ASSERT_TRUE(n <= 48 + (512 - sizeof(struct timeval)));
+        ASSERT_TRUE(n <= 48 + (512 - static_cast<int>(sizeof(struct timeval))));
       }
       va_end(backup_ap);
     }