Fix bug in stripComments

Summary: the bug is in InString state when we escape only '"' character. We should escape at least '\\' and '"'. This diffs simply escapes any character after '\\'.

Test Plan: folly unit tests, mcrouter unit tests

Reviewed By: stepan@fb.com

Subscribers: njormrod

FB internal diff: D1540836

folly/json.cpp

@@ -705,18 +705,21 @@ fbstring stripComments(StringPiece jsonC) {
           state = State::LineComment;
           ++i;
           continue;
-        } else if (s.startsWith("\"")) {
+        } else if (s[0] == '\"') {
           state = State::InString;
         }
         result.push_back(s[0]);
         break;
       case State::InString:
-        if (s.startsWith("\\\"")) {
+        if (s[0] == '\\') {
+          if (UNLIKELY(s.size() == 1)) {
+            throw std::logic_error("Invalid JSONC: string is not terminated");
+          }
           result.push_back(s[0]);
           result.push_back(s[1]);
           ++i;
           continue;
-        } else if (s.startsWith("\"")) {
+        } else if (s[0] == '\"') {
           state = State::None;
         }
         result.push_back(s[0]);
@@ -728,7 +731,7 @@ fbstring stripComments(StringPiece jsonC) {
         }
         break;
       case State::LineComment:
-        if (s.startsWith("\n")) {
+        if (s[0] == '\n') {
           // skip the line break. It doesn't matter.
           state = State::None;
         }

folly/test/json_test_data/commented.json

@@ -7,5 +7,6 @@
   */
   "test4": "foo /* bar", /* comment */
   "te//": "foo",
-  "te/*": "bar"
+  "te/*": "bar",
+  "\\\"": "\\" /* comment */
 }

folly/test/json_test_data/commented.json.exp

@@ -2,5 +2,6 @@
     "test": "foo",   "test2": "foo // bar",   
   "test4": "foo /* bar", 
   "te//": "foo",
-  "te/*": "bar"
+  "te/*": "bar",
+  "\\\"": "\\" 
 }