Fix use-after-free in Reader::parse(std::istream&) (#1665)

The istream overload stored the document in a local String then
passed raw pointers into it to parse(const char*, const char*),
which kept those pointers in begin_/end_. After parse() returned
the local String was destroyed, leaving begin_/end_ dangling.
Any subsequent call to getFormattedErrorMessages() would then
read freed memory.

Fix by reading the stream into the member document_ instead, matching
the behavior of parse(const std::string&).

Also document the lifetime requirement on parse(const char*, const
char*): the caller's buffer must outlive the Reader if error-reporting
methods are used after parsing.

Fixes #1623

include/json/reader.h

diff --git a/include/json/reader.h b/include/json/reader.h
index d745378..7aa2271 100644
--- a/include/json/reader.h
+++ b/include/json/reader.h
@@ -81,7 +81,10 @@
    * document.
    *
    * \param      beginDoc        Pointer on the beginning of the UTF-8 encoded
-   *                             string of the document to read.
+   *                             string of the document to read. The pointed-to
+   *                             buffer must outlive this Reader if error
+   *                             methods (e.g. getFormattedErrorMessages()) are
+   *                             called after parse() returns.
    * \param      endDoc          Pointer on the end of the UTF-8 encoded string
    *                             of the document to read.  Must be >= beginDoc.
    * \param[out] root            Contains the root value of the document if it

src/lib_json/json_reader.cpp

diff --git a/src/lib_json/json_reader.cpp b/src/lib_json/json_reader.cpp
index 3faa202..83743f7 100644
--- a/src/lib_json/json_reader.cpp
+++ b/src/lib_json/json_reader.cpp
@@ -88,15 +88,10 @@
 }
 
 bool Reader::parse(std::istream& is, Value& root, bool collectComments) {
-  // std::istream_iterator<char> begin(is);
-  // std::istream_iterator<char> end;
-  // Those would allow streamed input from a file, if parse() were a
-  // template function.
-
-  // Since String is reference-counted, this at least does not
-  // create an extra copy.
-  String doc(std::istreambuf_iterator<char>(is), {});
-  return parse(doc.data(), doc.data() + doc.size(), root, collectComments);
+  document_.assign(std::istreambuf_iterator<char>(is),
+                   std::istreambuf_iterator<char>());
+  return parse(document_.data(), document_.data() + document_.size(), root,
+               collectComments);
 }
 
 bool Reader::parse(const char* beginDoc, const char* endDoc, Value& root,