ldb: fix dump command to pad HEX output chars with 0.

Summary: The old code was omitting the 0 if the char is less than 16.

Test Plan:
Tried the following program:
int main() {
  unsigned char c = 1;
  printf("%X\n", c);
  printf("%02X\n", c);
  return 0;
}
The output is:
1
01

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D7437

util/ldb_cmd.cc

@@ -245,12 +245,12 @@ void DBDumper::DoCommand() {
       if (hex_output_) {
         std::string str = iter->key().ToString();
         for (unsigned int i = 0; i < str.length(); ++i) {
-          fprintf(stdout, "%X", str[i]);
+          fprintf(stdout, "%02X", (unsigned char)str[i]);
         }
         fprintf(stdout, " ==> ");
         str = iter->value().ToString();
         for (unsigned int i = 0; i < str.length(); ++i) {
-          fprintf(stdout, "%X", str[i]);
+          fprintf(stdout, "%02X", (unsigned char)str[i]);
         }
         fprintf(stdout, "\n");
       } else {