Fix potential overflow in CalculateSSTWriteHint() (#6212)

Summary:
level passed into ColumnFamilyData::CalculateSSTWriteHint() can be smaller than base_level in current version, which would cause overflow.
We see ubsan complains:

db/compaction/compaction_job.cc:1511:39: runtime error: load of value 4294967295, which is not a valid value for type 'Env::WriteLifeTimeHint'

and I hope this commit fixes it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6212

Test Plan: Run existing tests and see them to pass.

Differential Revision: D19168442

fbshipit-source-id: bf8fd86f85478ecfa7556db46dc3242de8c83dc9

db/column_family.cc

@@ -1306,6 +1306,10 @@ Env::WriteLifeTimeHint ColumnFamilyData::CalculateSSTWriteHint(int level) {
   // L1: medium, L2: long, ...
   if (level - base_level >= 2) {
     return Env::WLTH_EXTREME;
+  } else if (level < base_level) {
+    // There is no restriction which prevents level passed in to be smaller
+    // than base_level.
+    return Env::WLTH_MEDIUM;
   }
   return static_cast<Env::WriteLifeTimeHint>(level - base_level +
                             static_cast<int>(Env::WLTH_MEDIUM));