Fix broken test; some ldb commands can run without a db_

Summary:
It would appear our unit tests make use of code from ldb_cmd,
and don't always require a valid database handle.  D6855 was not aware
db_ could sometimes be NULL for such commands, and so it broke
reduce_levels_test.

This moves the check elsewhere to (at least) fix the 'ldb dump' case of
segfaulting when it couldn't open a database.

Test Plan: make check

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D6903

util/ldb_cmd.cc

@@ -126,6 +126,9 @@ void DBDumper::Help(std::string& ret) {
 }
 
 void DBDumper::DoCommand() {
+  if (!db_) {
+    return;
+  }
   // Parse command line args
   uint64_t count = 0;
   if (print_stats_) {

util/ldb_cmd.h

@@ -134,10 +134,6 @@ public:
       OpenDB();
     }
 
-   if (!db_) {
-     return;
-   }
-
     DoCommand();
     if (exec_state_.IsNotStarted()) {
       exec_state_ = LDBCommandExecuteResult::SUCCEED("");