Minor tweaks to StringAppend MergeOperator.

Summary:
I'm concerned about a random seg-fault that sometimes occurs when
running stringappend_test. I will investigate further. First, I am removing
stringappend_test from the regular release tests, and making some clean-ups
to the code.

Test Plan:
1. make stringappend_test
2. ./stringappend_test

Reviewers: haobo, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11313

Makefile

@@ -62,7 +62,6 @@ TESTS = \
 	auto_roll_logger_test \
 	filelock_test \
 	merge_test \
-	stringappend_test \
 	redis_test
 
 TOOLS = \

utilities/merge_operators/string_append/stringappend.cc

@@ -43,8 +43,6 @@ void StringAppendOperator::Merge(const Slice& key,
     new_value->append(1,delim_);
     new_value->append(value.data(), value.size());
   }
-
-  return;
 }
 
 const char* StringAppendOperator::Name() const  {

utilities/merge_operators/string_append/stringappend_test.cc

@@ -77,7 +77,7 @@ class StringLists {
 
     //NotFound is okay; just return empty (similar to std::map)
     //But network or db errors, etc, should fail the test (or at least yell)
-    if (s.ToString() != "NotFound: "){
+    if (!s.IsNotFound()) {
       std::cerr << "ERROR " << s.ToString() << std::endl;
     }