Fix analyze error on possible un-initialized value (#4937)

Summary:
The patch fixes the following analyze error by checking the return status of ParseInternalKey.
```
db/merge_helper.cc:306:23: warning: The right operand of '==' is a garbage value
    assert(kTypeMerge == orig_ikey.type);
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4937

Differential Revision: D13908506

Pulled By: maysamyabandeh

fbshipit-source-id: 68d7771e75519da3d4bd807fd231675ec12093f6

db/merge_helper.cc

@@ -138,7 +138,11 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
   // orig_ikey is backed by original_key if keys_.empty()
   // orig_ikey is backed by keys_.back() if !keys_.empty()
   ParsedInternalKey orig_ikey;
-  ParseInternalKey(original_key, &orig_ikey);
+  bool succ = ParseInternalKey(original_key, &orig_ikey);
+  assert(succ);
+  if (!succ) {
+    return Status::Corruption("Cannot parse key in MergeUntil");
+  }
 
   Status s;
   bool hit_the_next_user_key = false;