Replace exception by setting valid_ = false in DBIter::MergeValuesNewToOld()

Summary: Replace exception by setting valid_ = false in DBIter::MergeValuesNewToOld().

Test Plan:
Not sure if I am right at this, but it seems we currently don't have a good
way to test that code path as it requires dynamically set merge_operator = nullptr
at the time while Merge() is calling.

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29811

db/db_iter.cc

@@ -281,8 +281,9 @@ void DBIter::MergeValuesNewToOld() {
   if (!user_merge_operator_) {
     Log(InfoLogLevel::ERROR_LEVEL,
         logger_, "Options::merge_operator is null.");
-    throw std::logic_error("DBIter::MergeValuesNewToOld() with"
+    status_ = Status::InvalidArgument("user_merge_operator_ must be set.");
-                           " Options::merge_operator null");
+    valid_ = false;
+    return;
   }
 
   // Start the merge process by pushing the first operand