Fix SIGSEGV issue in universal compaction

Summary:
We saw SIGSEGV when set options.num_levels=1 in universal compaction
style. Dug into this issue for a while, and finally found the root cause (thank Haobo for discussion).

Test Plan: Add new unit test. It throws SIGSEGV without this change. Also run "make all check".

Reviewers: haobo, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13251

db/db_impl.cc

@@ -2126,7 +2126,7 @@ Status DBImpl::DoCompactionWork(CompactionState* compact) {
   }
 
   mutex_.Lock();
-  stats_[compact->compaction->level() + 1].Add(stats);
+  stats_[compact->compaction->output_level()].Add(stats);
 
   // if there were any unused file number (mostly in case of
   // compaction error), free up the entry from pending_putputs

db/db_test.cc

@@ -1803,6 +1803,39 @@ TEST(DBTest, UniversalCompactionSizeAmplification) {
   ASSERT_EQ(NumTableFilesAtLevel(0), 1);
 }
 
+TEST(DBTest, UniversalCompactionOptions) {
+  Options options = CurrentOptions();
+  options.compaction_style = kCompactionStyleUniversal;
+  options.write_buffer_size = 100<<10; //100KB
+  options.level0_file_num_compaction_trigger = 4;
+  options.num_levels = 1;
+  Reopen(&options);
+
+  Random rnd(301);
+  int key_idx = 0;
+
+  for (int num = 0;
+       num < options.level0_file_num_compaction_trigger;
+       num++) {
+    // Write 120KB (12 values, each 10K)
+    for (int i = 0; i < 12; i++) {
+      ASSERT_OK(Put(Key(key_idx), RandomString(&rnd, 10000)));
+      key_idx++;
+    }
+    dbfull()->TEST_WaitForCompactMemTable();
+
+    if (num < options.level0_file_num_compaction_trigger - 1) {
+      ASSERT_EQ(NumTableFilesAtLevel(0), num + 1);
+    }
+  }
+
+  dbfull()->TEST_WaitForCompact();
+  ASSERT_EQ(NumTableFilesAtLevel(0), 1);
+  for (int i = 1; i < options.num_levels ; i++) {
+    ASSERT_EQ(NumTableFilesAtLevel(i), 0);
+  }
+}
+
 TEST(DBTest, ConvertCompactionStyle) {
   Random rnd(301);
   int max_key_level_insert = 200;