Fixing a careless mistake in ldb

Summary: negation of the condition checked currently had to be checkd actually

Test Plan: make ldb; python ldb_test.py

Reviewers: sheki, dhruba

Reviewed By: sheki

Differential Revision: https://reviews.facebook.net/D9459

util/ldb_cmd.h

@@ -99,7 +99,7 @@ public:
 
   static string HexToString(const string& str) {
     string parsed;
-    if (str[0] == '0' && str[1] == 'x') {
+    if (str[0] != '0' || str[1] != 'x') {
       fprintf(stderr, "Invalid hex input %s.  Must start with 0x\n",
               str.c_str());
       throw "Invalid hex input";