Fix a rapidjson compile error in mac.

Summary:
This diff fixes the following compilation error in mac.
  ./third-party/rapidjson/reader.h:422:31: error: comparison of constant 256 with expression of type 'Ch' (aka 'char') is always true
        [-Werror,-Wtautological-constant-out-of-range-compare]
                                  if ((sizeof(Ch) == 1 || e < 256) && escape[(unsigned char)e])
                                                          ~ ^ ~~~
  1 error generated.

Test Plan: make db_test

Reviewers: haobo, sdong, igor, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19245

third-party/rapidjson/reader.h

@@ -419,7 +419,7 @@ private:
 			Ch c = s.Take();
 			if (c == '\\') {	// Escape
 				Ch e = s.Take();
-				if ((sizeof(Ch) == 1 || e < 256) && escape[(unsigned char)e])
+				if ((sizeof(Ch) == 1 || (int)e < 256) && escape[(unsigned char)e])
 					RAPIDJSON_PUT(escape[(unsigned char)e]);
 				else if (e == 'u') {	// Unicode
 					unsigned codepoint = ParseHex4(s);